Title: CSS Backgrounds and Borders Module Level 3
Status: ED
Implementation Report: https://wpt.fyi/results/css/css-backgrounds
Work Status: Testing
Shortname: css-backgrounds
Level: 3
Group: csswg
ED: https://drafts.csswg.org/css-backgrounds/
TR: https://www.w3.org/TR/css-backgrounds-3/
Previous Version: https://www.w3.org/TR/2024/CRD-css-backgrounds-3-20240311/
Previous Version: https://www.w3.org/TR/2020/CR-css-backgrounds-3-20201222/
Previous Version: https://www.w3.org/TR/2017/CR-css-backgrounds-3-20171017/
Previous version: https://www.w3.org/TR/2014/CR-css3-background-20140909/
Former Editor: Bert Bos, W3C, bert@w3.org, w3cid 3343
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Brad Kemper, Invited Expert, brad.kemper@gmail.com, w3cid 43245
Abstract: This draft contains the features of CSS relating to borders and backgrounds. The main extensions compared to level 2 are borders consisting of images, boxes with multiple backgrounds, boxes with rounded corners and boxes with shadows.
WPT Path Prefix: css/css-backgrounds/
WPT Display: closed
At risk: animatability of 'box-shadow'
At risk: applicability of 'border' and its longhands to [=ruby base containers=] and [=ruby annotation containers=]
Ignored Vars: width of background positioning area, width of background image, height of background positioning area, height of background image, X, X', Li, Ltop, Lbottom, Lleft, Lright, spread, Wside, Wleft, Wright, Wtop, Wbottom, spread distance
This subsection is not normative.
When elements are rendered according to the
CSS box model [[!CSS-BOX-3]],
each element is either not displayed at all,
or formatted as one or more rectangular boxes.
Each box has a rectangular [=content area=],
a band of [=padding=] around the content,
a [=border=] around the padding,
and a [=margin=] outside the border.
(The margin may actually be negative,
but margins have no influence on the background and border.)
The various areas and edges of a typical box.
(This diagram is explained in the CSS Box Model Module [[!CSS-BOX-3]].)
The properties of this module deal with the decoration of the [=border area=]
and with the background of the [=content area|content=], [=padding area|padding=], and [=border area|border=] areas.
Additionally the box may be given a “drop-shadow” effect
with the 'box-shadow' property.
If an element is broken into multiple [=box fragments=],
'box-decoration-break' [[CSS-BREAK-3]] defines
how the borders and background are divided over the various fragments.
(An element can result in more than one fragment if it is broken
at the end of a line, at the end of a column or at the end of a page;
and continued in the next line, column or page.)
The relative stacking order of backgrounds, borders, and shadows
is given in this module.
For how these layers interact with other rendered content,
see Appendix E “Elaborate description of Stacking Contexts” in [[!CSS2]].
Module Interactions
This module replaces and extends the background and border features
defined in [[!CSS2]] sections 8.5 and 14.2.
All properties in this module apply to the ''::first-letter'' [=pseudo-element=].
The background properties
and border-radius properties also apply
to the ''::first-line'' [=pseudo-element=].
The UA may (but is not required to)
apply the 'border-image' or 'box-shadow' properties to ''::first-line''.
The UA must not
apply the border-color/style/width properties
to ''::first-line''.
[[!CSS2]]
Value Definitions
This specification follows the CSS property definition conventions from [[!CSS2]]
using the value definition syntax from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
For example, combining with CSS Images
allows for using CSS gradients as 'background-image' or 'border-image' values.
[[CSS-IMAGES-3]]
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords as their property value.
For readability they have not been repeated explicitly.
Backgrounds
Each box has a background layer that may be fully transparent (the default),
or filled with a color and/or one or more images.
The background properties specify what color ('background-color')
and images ('background-image') to use,
and how they are sized, positioned, tiled, etc.
The background properties are not inherited,
but the parent box's background will shine through by default
because of the initial ''transparent'' value on 'background-color'.
Layering Multiple Background Images
The background of a box can have multiple
background image layers.
The number of layers is determined by
the number of comma-separated values in the 'background-image' property.
Note that a value of ''background-image/none'' still creates a layer.
background-none-none-and-color.html
order-of-images.htm
scroll-positioned-multiple-background-images.html
Each of the [=background images=] is sized, positioned, and tiled
according to the corresponding value in the other background properties.
The lists are matched up from the first value:
excess values at the end are not used.
If a property doesn't have enough comma-separated values
to match the number of layers,
the [=UA=] must calculate its [=used value=]
by repeating the list of values until there are enough.
For example, this set of declarations:
background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left, bottom right;
background-origin: border-box, content-box;
background-repeat: no-repeat;
has exactly the same effect as this set,
with the extra position dropped
and the missing values for 'background-origin' and 'background-repeat'
filled in (emphasized for clarity):
background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left;
background-origin: border-box, content-box, border-box;
background-repeat: no-repeat, no-repeat, no-repeat;
The first image in the list is the [=layer=] closest to the user,
the next one is painted behind the first, and so on.
The background color, if present,
is painted below all of the other [=layers=].
Note: The border-image properties
can also define a background image,
which, if present, is painted on top of
the background [=layers=] created by the background properties.
Base Color: the 'background-color' property
Name: background-color
Value: <>
Initial: transparent
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: computed color
Animation type: by computed value
animations/background-color-animation-backdrop-infinite-duration-crash.html
animations/background-color-animation-element-not-visible-at-current-viewport.html
animations/background-color-animation-fallback-additive-keyframe.html
animations/background-color-animation-fallback-missing-0-percent.html
animations/background-color-animation-fallback-missing-100-percent.html
animations/background-color-animation-fallback-replace.html
animations/background-color-animation-field-crash.html
animations/background-color-animation-fragmented.html
animations/background-color-animation-half-opaque.html
animations/background-color-animation-in-body.html
animations/background-color-animation-non-empty-no-draw-crash.html
animations/background-color-animation-non-zero-size-element-change-to-zero.html
animations/background-color-animation-pseudo-element.html
animations/background-color-animation-removed-element-crash.html
animations/background-color-animation-single-keyframe.html
animations/background-color-animation-three-keyframes1.html
animations/background-color-animation-three-keyframes2.html
animations/background-color-animation-three-keyframes3.html
animations/background-color-animation-will-change-contents.html
animations/background-color-animation-with-blur.html
animations/background-color-animation-with-images.html
animations/background-color-animation-with-mask.html
animations/background-color-animation-with-table1.html
animations/background-color-animation-with-table2.html
animations/background-color-animation-with-table3.html
animations/background-color-animation-with-table4.html
animations/background-color-animation-with-zero-playbackRate.html
animations/background-color-animation-zero-size-element-change-to-non-zero.html
animations/background-color-animation-zero-size-element.html
animations/background-color-animation.html
animations/background-color-interpolation.html
animations/background-color-scroll-into-viewport.html
animations/background-color-transition-colormix.html
animations/background-color-transition-currentcolor.html
animations/background-color-transition.html
animations/background-color-transparent-animation-in-body.html
animations/invalidation/background-color-animation-with-zero-alpha.html
animations/invalidation/background-color-transition-obscured.html
animations/invalidation/background-color-transition-with-delay.html
animations/invalidation/background-color-transition-with-initially-transparent.html
animations/two-background-color-animation-diff-length1.html
animations/two-background-color-animation-diff-length2.html
animations/two-background-color-animation-diff-length3.html
background-none-none-and-color.html
background-color-body-propagation-001.html
background-color-body-propagation-002.html
background-color-body-propagation-003.html
background-color-body-propagation-004.html
background-color-body-propagation-005.html
background-color-body-propagation-006.html
background-color-body-propagation-007.html
background-color-body-propagation-008.html
background-color-body-propagation-009.html
background-color-clip.html
background-color-root-propagation-001.html
background-color-root-propagation-002.html
bg-color-with-gradient.html
child-move-reveals-parent-background.html
color-mix-currentcolor-background-repaint-parent.html
color-mix-currentcolor-background-repaint.html
hidpi/simple-bg-color.html
inheritance.sub.html
inline-background-rtl-001.html
parsing/background-color-computed.html
parsing/background-color-invalid.html
parsing/background-color-valid.html
color-behind-images.htm
This property sets the background color of a box.
This color is drawn behind any background images.
The [=background color=] is clipped
according to the 'background-clip' value
associated with the bottom-most [=background image layer=].
Image Sources: the 'background-image' property
Name: background-image
Value: <>#
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item either an <> or the keyword ''background-image/none''
Animation type: discrete
background-image-001.html
background-image-002.html
background-image-003.html
background-image-004.html
background-image-005.html
background-image-006.html
background-image-007.html
background-image-centered-with-border-radius.html
background-image-centered.html
background-image-cors-no-reload.html
background-image-cover-zoomed-1.html
background-image-first-letter.html
background-image-first-line.html
background-image-gradient-currentcolor-conic-repaint.html
background-image-gradient-currentcolor-linear-repaint.html
background-image-gradient-currentcolor-radial-repaint.html
background-image-gradient-currentcolor-visited.html
background-image-gradient-interpolation-repaint.html
background-image-large-with-auto.html
background-image-none-gradient-repaint.html
background-image-shared-stylesheet.html
background-image-table-cells-straddling-no-repeat.html
background-image-table-cells-zoomed.html
background-image-with-border-radius-fidelity.html
animations/background-image-interpolation.html
inheritance.sub.html
parsing/background-image-computed.sub.html
parsing/background-image-invalid.html
parsing/background-image-valid.html
This property specifies the background image(s) of an element.
Images are drawn with the first specified one on top (closest to the user)
and each subsequent image behind the previous one.
The property's value is given as a comma-separated list
of <> values where
<> = <> | none
A value of none
counts as a [=background image layer=] but draws nothing.
An image that is empty (zero width or zero height),
that fails to download,
or that cannot be displayed
(e.g., because it is not in a supported image format)
likewise counts as a [=layer=] but draws nothing.
See [[#layering]] for how 'background-image' interacts
with other comma-separated background properties
to form each [=background image layer=].
When setting a background image,
authors should also specify a 'background-color'
that will preserve contrast with the text
for when the image is unavailable.
For accessibility reasons,
authors should not use background images
as the sole method of conveying important information.
See Web Content Accessibility Guideline F3 [[WCAG20]].
Images are not accessible in non-graphical presentations,
and background images specifically
might be turned off in high-contrast display modes.
Note: Stylistic foreground images can be provided in CSS
with the 'content' property.
Semantically-important foreground images should be provided
in the document markup, e.g. with the <img> tag in HTML.
Note: Media fragments
can be used to display a portion of an image.
The CSS Images module
will provide fallback syntax for image formats
and include additional controls for image display.
Some examples specifying background images:
html { background-image: url("marble.svg") }
p { background-image: none }
div { background-image: url(tl.png), url(tr.png) }
main { background-image: radial-gradient(at bottom right, transparent, white); }
Implementations may optimize
by not downloading and drawing images that are not visible
(e.g., because they are behind other, fully opaque images).
Tiling Images: the 'background-repeat' property
Name: background-repeat
Value: <>#
Initial: repeat
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a pair of keywords, one per dimension
Animation type: discrete
animations/discrete-no-interpolation.html
background-repeat-round-1a.html
background-repeat-round-1b.html
background-repeat-round-1c.html
background-repeat-round-1d.html
background-repeat-round-1e.html
background-repeat-round-2.html
background-repeat-round-3.html
background-repeat-round-4.html
background-repeat-space-10.html
background-repeat-space-1a.html
background-repeat-space-1b.html
background-repeat-space-1c.html
background-repeat-space-2.html
background-repeat-space-3.html
background-repeat-space-4.html
background-repeat-space-5.html
background-repeat-space-6.html
background-repeat-space-7.html
background-repeat-space-8.html
background-repeat-space-9.html
background-repeat/background-repeat-no-repeat.xht
background-repeat/background-repeat-repeat-x.xht
background-repeat/background-repeat-repeat-y.xht
background-repeat/background-repeat-round-roundup.xht
background-repeat/background-repeat-round.xht
background-repeat/background-repeat-space.xht
background-repeat/gradient-repeat-spaced-with-borders.html
inheritance.sub.html
parsing/background-repeat-computed.html
parsing/background-repeat-invalid.html
parsing/background-repeat-valid.html
subpixel-repeat-no-repeat-mix.html
This property specifies how [=background images=] are tiled
after they have been [[#background-size|sized]]
and [[#background-position|positioned]].
The property's value is given as a comma-separated list
of <> values where
If a <> value has two keywords,
the first one applies to the horizontal axis,
the second to the vertical one,
as follows:
repeat
The [=image=] is repeated in this direction as often as needed
to cover the [=background painting area=].
space
The [=image=] is repeated as often as will fit
within the [=background positioning area=] without being clipped,
and then the images are spaced out to fill the area.
The first and last images touch the edges of the area.
If the [=background painting area=] is larger
than the [=background positioning area=],
then the pattern repeats to fill the background painting area.
The value of 'background-position' for this direction is ignored
unless there is not enough space
for two copies of the image in this axis,
in which case only one image is placed,
and 'background-position' determines its position in this axis.
round
The [=image=] is repeated as often as will fit within the
[=background positioning area=]. If it doesn't
fit a whole number of times, it is rescaled so that it does.
See the formula under 'background-size'.
If the [=background painting area=] is larger than the background positioning area, then
the pattern repeats to fill the background painting area.
no-repeat
The [=image=] is placed once and not repeated in this direction.
Unless one of the two keywords is ''no-repeat'',
the whole [=background painting area=] will be tiled,
i.e., not just one vertical strip and one horizontal strip.
body {
background: white url("pendant.png");
background-repeat: repeat-y;
background-position: center;
}
The effect of ''repeat-y'':
One copy of the background image is centered,
and other copies are put above and below it
to make a vertical band behind the element.
body {
background-image: url(dot.png) white;
background-repeat: space
}
The effect of ''background-repeat/space'':
the image of a dot is tiled to cover the whole background
and the images are equally spaced.
See [[#layering]] for how 'background-repeat' interacts
with other comma-separated background properties
to form each [=background image layer=].
Affixing Images: the 'background-attachment' property
Name: background-attachment
Value: <>#
Initial: scroll
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item the keyword as specified
Animation type: discrete
animations/discrete-no-interpolation.html
background-attachment-350.html
background-attachment-353.html
background-attachment-fixed-block-002.html
background-attachment-fixed-border-radius-offset.html
background-attachment-fixed-inline-002.html
background-attachment-fixed-inline-scrolled.html
background-attachment-fixed-inside-transform-1.html
background-attachment-local-block-002.html
background-attachment-local-hidden.html
background-attachment-local/attachment-local-clipping-color-1.html
background-attachment-local/attachment-local-clipping-color-2.html
background-attachment-local/attachment-local-clipping-color-3.html
background-attachment-local/attachment-local-clipping-color-4.html
background-attachment-local/attachment-local-clipping-color-5.html
background-attachment-local/attachment-local-clipping-color-6.html
background-attachment-local/attachment-local-clipping-image-1.html
background-attachment-local/attachment-local-clipping-image-2.html
background-attachment-local/attachment-local-clipping-image-3.html
background-attachment-local/attachment-local-clipping-image-4.html
background-attachment-local/attachment-local-clipping-image-5.html
background-attachment-local/attachment-local-clipping-image-6.html
background-attachment-local/attachment-local-positioning-2.html
background-attachment-local/attachment-local-positioning-3.html
background-attachment-local/attachment-local-positioning-4.html
background-attachment-local/attachment-local-positioning-5.html
background-attachment-local/attachment-scroll-positioning-1.html
background-attachment-margin-root-001.html
background-attachment-margin-root-002.html
inheritance.sub.html
local-attachment-content-box-scroll.html
parsing/background-attachment-computed.html
parsing/background-attachment-invalid.html
parsing/background-attachment-valid.html
If [=background images=] are specified,
this property specifies whether they are
fixed with regard to the [=viewport=] (''fixed'')
or scroll along with the box (''scroll'')
or its contents (''local'').
The property's value is given as a comma-separated list
of <> keywords where
<> = scroll | fixed | local
fixed
The background is fixed with regard to the viewport.
In [=paged media=] where there is no viewport,
a ''fixed'' background is fixed with respect to
the page box
and therefore replicated on every page.
Note: There is only one viewport per view.
Even if an box is a [=scroll container=],
a ''fixed'' background doesn't move with the box.
local
The background is fixed with regard to the box's contents:
if the box has a scrolling mechanism,
the background scrolls with the box's contents,
and the [=background painting area=] and [=background positioning area=]
are relative to the [=scrollable overflow area=] of the box
rather than to the border framing them.
Because the [=scrollable overflow area=]
does not include the [=border area=],
for [=scroll containers=] the ''background-clip/border-box'' value of 'background-clip'
may be treated the same as ''background-clip/padding-box''.
scroll
The background is fixed with regard to the box itself
and does not scroll with its contents.
(It is effectively attached to the box's border.)
Even if the image is fixed,
it is still only visible when it is in the [=background painting area=] of the box
or otherwise unclipped.
(See [[#special-backgrounds]] for the cases when
background images are not clipped.)
Thus, unless the image is tiled, it may be invisible.
This example creates an infinite vertical band
that remains “glued” to the viewport when the document is scrolled.
body {
background: red url("pendant.gif");
background-repeat: repeat-y;
background-attachment: fixed;
}
Note: User agents that do not support ''fixed'' backgrounds
(for example due to limitations of the hardware platform)
will ignore declarations
with the keyword ''fixed''.
For example:
body {
/* For all UAs: */
background: white url(paper.png) scroll;
/* For UAs that do fixed backgrounds: */
background: white url(ledger.png) fixed;
}
h1 {
/* For all UAs: */
background: silver;
/* For UAs that do fixed backgrounds: */
background: url(stripe.png) fixed, white url(ledger.png) fixed;
}
See [[#layering]] for how 'background-attachment' interacts
with other comma-separated background properties
to form each [=background image layer=].
Positioning Images: the 'background-position' property
Name: background-position
Value: <>#
Initial: 0% 0%
Applies to: all elements
Inherited: no
Percentages: refer to size of [=background positioning area=]
minus size of background image; see text
Computed value: list,
each item a pair of offsets (horizontal and vertical) from the top left origin
each given as a computed <> value
Animation type: repeatable list
animations/background-position-composition.html
animations/background-position-interpolation.html
background-position-calc-minmax-001.html
background-position-negative-percentage-comparison-002.html
background-position-negative-percentage-comparison.html
background-position-three-four-values.html
background-position/background-position-bottom-right-repeat-round.html
background-position/background-position-right-in-body.html
inheritance.sub.html
parsing/background-position-computed.html
parsing/background-position-invalid.html
parsing/background-position-valid.html
If [=background images=] have been specified,
this property specifies their initial position
(after any resizing)
within their corresponding [=background positioning area=].
The property's value is given as a comma-separated list
of <> values where
<> = [
[ left | center | right | top | bottom | <> ]
|
[ left | center | right | <> ]
[ top | center | bottom | <> ]
|
[ center | [ left | right ] <>? ] &&
[ center | [ top | bottom ] <>? ]
]
If only one value is specified,
the second value is assumed to be ''center''.
If two values are given,
a <> as the first value represents
the horizontal position (or offset)
and a <> as the second value represents
the vertical position (or offset).
The <> values here represent
an offset of the top left corner of the background image
from the top left corner of the [=background positioning area=].
Note: A pair of keywords can be reordered,
while a combination of keyword and length or percentage cannot.
So ''center left'' is valid while ''50% left'' is not.
If three or four values are given,
then each <> represents an offset
and must be preceded by a keyword,
which specifies from which edge the offset is given.
For example, ''background-position: bottom 10px right 20px''
represents a ''10px'' vertical offset up from the bottom edge
and a ''20px'' horizontal offset leftward from the right edge.
If three values are given,
the missing offset is assumed to be zero.
Positive values represent an offset inward
from the edge of the [=background positioning area=].
Negative values represent an offset outward
from the edge of the [=background positioning area=].
The following declarations give the stated (horizontal, vertical)
offsets from the top left corner:
background-position: left 10px top 15px; /* 10px, 15px */
background-position: left top ; /* 0px, 0px */
background-position: 10px 15px; /* 10px, 15px */
background-position: left 15px; /* 0px, 15px */
background-position: 10px top ; /* 10px, 0px */
background-position: left top 15px; /* 0px, 15px */
background-position: left 10px top ; /* 10px, 0px */
<>
A percentage for the horizontal offset is relative to
(width of [=background positioning area=] - width of [=background image=]).
A percentage for the vertical offset is relative to
(height of [=background positioning area=] - height of [=background image=]),
where the size of the image is the size given by 'background-size'.
For example, with a value pair of ''0% 0%'',
the upper left corner of the image is aligned with
the upper left corner of, usually, the box's [=padding edge=].
A value pair of ''100% 100%'' places
the lower right corner of the image
in the lower right corner of the area.
With a value pair of ''75% 50%'',
the point 75% across and 50% down the image
is to be placed at the point 75% across and 50% down the area.
Diagram of the meaning of ''background-position: 75% 50%''.
<>
A length value gives a fixed length as the offset.
For example, with a value pair of ''2cm 1cm'',
the upper left corner of the image is placed
2cm to the right and 1cm below
the upper left corner of the [=background positioning area=].
''top''
Computes to ''0%'' for the vertical position if one or two values are given,
otherwise specifies the top edge as the origin for the next offset.
''right''
Computes to ''100%'' for the horizontal position if one or two values are given,
otherwise specifies the right edge as the origin for the next offset.
''bottom''
Computes to ''100%'' for the vertical position if one or two values are given,
otherwise specifies the bottom edge as the origin for the next offset.
''left''
Computes to ''0%'' for the horizontal position if one or two values are given,
otherwise specifies the left edge as the origin for the next offset.
''center''
Computes to ''50%'' (''left 50%'') for the horizontal position
if the horizontal position is not otherwise specified,
or ''50%'' (''top 50%'') for the vertical position if it is.
The following 'background' shorthand declarations use keywords
to set 'background-position' to the stated percentage values.
body { background: url("banner.jpeg") right top } /* 100% 0% */
body { background: url("banner.jpeg") top center } /* 50% 0% */
body { background: url("banner.jpeg") center } /* 50% 50% */
body { background: url("banner.jpeg") bottom } /* 50% 100% */
In the example below, the (single) image is placed
in the lower-right corner of the viewport.
Background positions can also be relative to other corners than the top left.
For example, the following puts the background image
10px from the bottom and 3em from the right:
background-position: right 3em bottom 10px
See [[#layering]] for how 'background-position' interacts
with other comma-separated background properties
to form each [=background image layer=].
Serialization of 'background-position' values
The [=specified value=] and [=computed value=] of the <> type
serialize exactly as defined in [[CSS-VALUES-4]] for <>.
For 3-value productions
(which are not valid in <>),
the [=specified value=] serialization
is identical to the equivalent 4-value syntax
except that the omitted offset remains omitted.
Painting Area: the 'background-clip' property
Name: background-clip
Value: <>#
Initial: border-box
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a keyword as specified
Animation type: repeatable list
animations/discrete-no-interpolation.html
background-clip-001.html
background-clip-002.html
background-clip-003.html
background-clip-004.html
background-clip-005.html
background-clip-006.html
background-clip-007.html
background-clip-008.html
background-clip-009.html
background-clip-010.html
background-clip-color-repaint.html
background-clip-color.html
background-clip-content-box-001.html
background-clip-content-box-002.html
background-clip-content-box-with-border-radius-002.html
background-clip-content-box-with-border-radius-003.html
background-clip-padding-box-with-border-radius-002.html
background-clip-padding-box-with-border-radius-003.html
background-clip-padding-box-001.html
background-clip-padding-box-with-border-radius.html
background-clip/clip-border-area-background-geometry.html
background-clip/clip-border-area-border-image.html
background-clip/clip-border-area-border-on-top.html
background-clip/clip-border-area-box-decoration-break.html
background-clip/clip-border-area-multiple-backgrounds.html
background-clip/clip-border-area-on-body-not-propagated-to-root.html
background-clip/clip-border-area-on-body-propagated-to-root.html
background-clip/clip-border-area-on-root.html
background-clip/clip-border-area.html
background-clip/clip-text-ellipsis.html
background-clip/clip-text-on-body-not-propagated-to-root.html
background-clip/clip-text-on-body-propagated-to-root.html
background-clip/clip-text-on-root.html
background-clip/clip-text-out-of-flow-child.html
background-clip/clip-text-text-decorations.html
background-clip/clip-text-text-emphasis.html
background-clip/clip-rounded-corner.html
background-clip/clip-text-animated-text.html
background-clip/clip-text-dynamic-2.html
background-clip/clip-text-flex.html
background-clip/clip-text-multi-line.html
background-clip_padding-box.html
background-paint-order-001.html
background-rounded-image-clip-001.html
background-rounded-image-clip-002.html
css3-background-clip-border-box.html
css3-background-clip-content-box.html
css3-background-clip-padding-box.html
css3-background-clip.html
inheritance.sub.html
local-attachment-content-box-scroll.html
parsing/background-clip-computed.html
parsing/background-clip-invalid.html
parsing/background-clip-valid.html
Determines the background painting area,
which determines the area within which the background is painted.
Values have the following meanings:
border-box
The background is painted within (clipped to) the
[=border box=].
padding-box
The background is painted within (clipped to) the
[=padding box=].
content-box
The background is painted within (clipped to) the
[=content box=].
Note: The root element has a different [=background painting area=]
and thus the 'background-clip' property has no effect when specified on it.
See [[#special-backgrounds]].
Note: The background is always drawn behind the border, if any.
See “Elaborate description of Stacking Contexts” in [[!CSS2]] Appendix E.
See [[#corner-shaping]] for how 'border-radius' affects
the shape of the [=background painting area=].
See [[#layering]] for how 'background-clip' interacts
with other comma-separated background properties
to form each [=background image layer=].
Positioning Area: the 'background-origin' property
Name: background-origin
Value: <>#
Initial: padding-box
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a keyword as specified
Animation type: repeatable list
animations/discrete-no-interpolation.html
animations/background-position-origin-interpolation.html
background-gradient-subpixel-fills-area.html
background-origin-001.html
background-origin-002.html
background-origin-003.html
background-origin-004.html
background-origin-005.html
background-origin-006.html
background-origin-007.html
background-origin-008.html
background-origin/origin-border-box.html
background-origin/origin-border-box_with_position.html
background-origin/origin-border-box_with_radius.html
background-origin/origin-border-box_with_size.html
background-origin/origin-content-box.html
background-origin/origin-content-box_with_position.html
background-origin/origin-content-box_with_radius.html
background-origin/origin-content-box_with_size.html
background-origin/origin-padding-box.html
background-origin/origin-padding-box_with_position.html
background-origin/origin-padding-box_with_radius.html
background-origin/origin-padding-box_with_size.html
css3-background-origin-border-box.html
css3-background-origin-content-box.html
css3-background-origin-padding-box.html
inheritance.sub.html
parsing/background-origin-computed.html
parsing/background-origin-invalid.html
parsing/background-origin-valid.html
This property determines the background positioning area:
the area within which any background images are positioned.
For elements rendered as multiple [=box fragments=]
(e.g., inline boxes on several lines, boxes on several pages),
specifies which boxes 'box-decoration-break' [[CSS-BREAK-3]] operates on
to determine the background positioning area(s).
padding-box
The position is relative to the [=padding box=].
(For single boxes ''0 0'' is the upper left corner of the padding edge,
''100% 100%'' is the lower right corner.)
border-box
The position is relative to the [=border box=].
content-box
The position is relative to the [=content box=].
If the 'background-attachment' value for this [=layer=] is ''fixed'',
then this property has no effect:
in this case the [=background positioning area=] is
the [=initial containing block=].
Note: If 'background-clip' is ''background-clip/padding-box'',
'background-origin' is ''background-origin/border-box'',
'background-position' is ''top left'' (the initial value),
and the element has a non-zero border,
then the top and left edges of the [=background image=] will be clipped.
See [[#layering]] for how 'background-origin' interacts
with other comma-separated background properties
to form each [=background image layer=].
Sizing Images: the 'background-size' property
Name: background-size
Value: <>#
Initial: auto
Applies to: all elements
Inherited: no
Percentages: see text
Computed value: list,
each item a pair of sizes (one per axis)
each represented as either a keyword or a computed <> value
Animation type: repeatable list
Scale the image, while preserving its [=natural aspect ratio=] (if any),
to the largest size such that both its width and its height
can fit inside the [=background positioning area=].
cover
Scale the image, while preserving its [=natural aspect ratio=] (if any),
to the smallest size such that both its width and its height
can completely cover the [=background positioning area=].
[ <>
| auto ]{1,2}
The first value gives the width of the corresponding image,
the second value its height.
If only one value is given
the second is assumed to be ''background-size/auto''.
A <> is relative to the [=background positioning area=].
An ''background-size/auto'' value for one dimension
is resolved by using the image's [=natural aspect ratio=]
and the size of the other dimension,
or failing that, using the image's [=natural size=],
or failing that, treating it as ''100%''.
If both values are ''background-size/auto''
then the [=natural width=] and/or [=natural height|height=] of the image
should be used, if any,
the missing dimension (if any) behaving as ''background-size/auto''
as described above.
If the image has neither [=natural size=],
its size is determined as for ''contain''.
Negative values are invalid.
Here are some examples.
The first example stretches the background image
independently in both dimensions
to completely cover the content area:
The second example stretches the image
so that exactly two copies fit horizontally.
The aspect ratio is preserved:
p {
background-image: url(tubes.png);
background-size: 50% auto;
background-origin: border-box }
This example forces the background image to be 15 by 15 pixels:
p {
background-size: 15px 15px;
background-image: url(tile.png)}
This example uses the image's natural size.
Note that this is the only possible behavior in CSS level 1 and 2.
body {
background-size: auto; /* default */
background-image: url(flower.png) }
The following example rounds the height of the image to 33.3%,
up from the specified value of 30%.
At 30%, three images would fit entirely and a fourth only partially.
After rounding, three images fit exactly.
The width of the image is 20% of the background positioning area width
and is not rounded.
If 'background-repeat' is ''background-repeat/round''
for one (or both) dimensions,
there is a second step.
The UA must scale the image in that dimension (or both dimensions)
so that it fits a whole number of times in the [=background positioning area=].
In the case of the width (height is analogous):
If X ≠ 0 is the width of the image after step one
and W is the width of the background positioning area,
then the rounded width
X' = W / round(W / X)
where round() is a function that returns the nearest natural number
(integer greater than zero).
If 'background-repeat' is ''background-repeat/round'' for one dimension only
and if 'background-size' is ''background-size/auto'' for the other dimension,
then there is a third step:
that other dimension is scaled so that the original aspect ratio is restored.
In this example the background image is shown at its natural size:
div {
background-image: url(image1.png);
background-repeat: repeat; /* default */
background-size: auto } /* default */
In the following example, the background is shown with a width of
3em and its height is scaled proportionally to keep the original
aspect ratio:
In the following example,
the background is shown with a width of approximately 3em:
scaled so that it fits a whole number of times in the width of the background.
The height is scaled proportionally to keep the original aspect ratio:
div {
background-image: url(image3.png);
background-repeat: round repeat;
background-size: 3em auto }
In the following example, the background image is shown
with a width of 3em and a height that is either
the height corresponding to that width at the original aspect ratio
or slightly less:
div {
background-image: url(image4.png);
background-repeat: repeat round;
background-size: 3em auto }
In the following example,
the background image is shown with a height of approximately 4em:
scaled slightly so that it fits a whole number of times in the background height.
The width is the approximately the width that corresponds to
a 4em height at the original aspect ratio:
scaled slightly so that it fits a whole number of times
in the background width.
div {
background-image: url(image5.png);
background-repeat: round;
background-size: auto 4em }
If the background image's width or height resolves to zero,
this causes the image not to be displayed.
(The effect is the same as if it had been a transparent image.)
See [[#layering]] for how 'background-size' interacts
with other comma-separated background properties
to form each [=background image layer=].
Serialization of 'background-size' values
The [=specified value=] and [=computed value=] of the <> type
always serialize as two values, even when the second value is ''background-size/auto'',
due to the [[cssom#serializing-css-values|shortest, most backwards-compatible serialization principle]].
Backgrounds Shorthand: the 'background' property
Name: background
Value: <>#? , <>
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
background-331.html
background-332.html
background-333.html
background-334.html
background-335.html
background-336.html
parsing/background-computed.html
parsing/background-invalid.html
parsing/background-shorthand-serialization.html
parsing/background-valid.html
The 'background' property is a [=shorthand property=]
for setting most background properties at the same place in the style sheet.
The number of comma-separated items defines the number of [=background image layers=].
Given a valid declaration, for each layer
the shorthand first sets the corresponding value of each of
'background-image',
'background-position',
'background-size',
'background-repeat',
'background-origin',
'background-clip'
and 'background-attachment' to
that property's [=initial value=],
then assigns any explicit values
specified for this layer in the declaration.
Finally 'background-color' is set to the specified color, if any,
else set to its initial value.
This property's value is given as a comma-separated list
of values where
Note: A color is permitted in <>, but not in <>.
If one <> value is present
then it sets both 'background-origin' and 'background-clip' to that value.
If two values are present,
then the first sets 'background-origin' and the second 'background-clip'.
In the first rule of the following example, only a value for
'background-color' has been given and the
other individual properties are set to their initial values. In the
second rule, many individual properties have been specified.
body { background: red }
p { background: url("chess.png") 40% / 10em gray
round fixed border-box; }
The following example shows how a both
a background color (''#CCC'') and a background image (''url(metal.jpg)'')
are set.
The image is rescaled to the full width of the element:
E { background: #CCC url("metal.jpg") top left / 100% auto no-repeat}
Another example shows equivalence:
div { background: padding-box url(paper.jpg) white center }
div {
background-color: white;
background-image: url(paper.jpg);
background-repeat: repeat;
background-attachment: scroll;
background-position: center;
background-clip: padding-box;
background-origin: padding-box;
background-size: auto auto }
The following declaration with multiple, comma-separated values
background: url(a.png) top left no-repeat,
url(b.png) center / 100% 100% no-repeat,
url(c.png) white;
is equivalent to
background-image: url(a.png), url(b.png), url(c.png);
background-position: top left, center, top left;
background-repeat: no-repeat, no-repeat, repeat;
background-clip: border-box, border-box, border-box;
background-origin: padding-box, padding-box, padding-box;
background-size: auto auto, 100% 100%, auto auto;
background-attachment: scroll, scroll, scroll;
background-color: white;
Backgrounds of Special Elements
The document canvas
is the infinite surface over which the document is rendered. [[!CSS2]]
Since no element corresponds to the canvas,
in order to allow styling of the canvas
CSS propagates the background of the [=root element=]
(or, in the case of HTML, the <body> element)
as described below.
However, if the element whose background would be used for the canvas
is ''display: none'',
then the [=canvas background=] is transparent.
If the [=canvas background=] is not opaque,
the canvas surface below it shows through.
The texture of the [=canvas surface=] is UA-dependent
(but is typically an opaque white).
The Canvas Background and the Root Element
The background of the [=root element=] becomes the canvas background
and its [=background painting area=] extends
to cover the entire canvas.
However, any images are sized and positioned relative to the root element’s box
as if they were painted for that element alone.
(In other words, the
background positioning area
is determined as for the root element.)
The root element does not paint this background again,
i.e., the [=used value=] of its background is ''transparent''.
background-margin-iframe-root.html
background-margin-root.html
background-margin-transformed-root.html
background-margin-will-change-root.html
The Canvas Background and the HTML <body> Element
For documents whose root element is
an HTML HTML element
or an XHTML html element [[!HTML]]:
if the [=computed value=] of 'background-image' on the [=root element=]
is ''background-image/none'' and its 'background-color' is ''transparent'',
user agents must instead propagate
the [=computed values=] of the background properties
from that element's first
HTML BODY or XHTML body
child element.
The [=used values=]
of that BODY element's background properties
are their [=initial values=],
and the propagated values are treated
as if they were specified on the root element.
It is recommended that authors of HTML documents specify the canvas background
using the BODY element
rather than the HTML element.
document-canvas-remove-body.html
Note: Using [=containment=] disables
this special handling of the HTML <{body}> element.
See the [[CSS-CONTAIN-1#contain-property]] for details.
According to these rules, the canvas underlying the following HTML document
will have a “marble” background:
<!DOCTYPE html PUBLIC '-//W3C//DTD HTML 4.0//EN'
>
<html>
<head>
<title>Setting the canvas background</title>
<style type="text/css">
body { background: url("http://example.org/marble.png") }
</style>
</head>
<body>
<p>My background is marble.</p>
</body>
</html>
The ''::first-line'' Pseudo-element‘s Background
The ''::first-line'' pseudo-element
is like an inline-level element
for the purposes of the background
(see section 5.12.1 of [[!CSS2]]).
That means, e.g., that in a left-justified first line,
the background does not necessarily extend
all the way to the right edge.
linear-gradient-currentcolor-first-line.html
Borders
The [=border=] can either be a predefined style
(solid line, double line, dotted line, pseudo-3D border, etc.)
or it can be an image.
In the former case, various properties define
the style ('border-style'),
color ('border-color'),
and thickness ('border-width')
of the border.
Line Colors: the 'border-color' properties
Name: border-top-color, border-right-color, border-bottom-color, border-left-color
Value: <>
Initial: currentColor
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Logical property group: border-color
Percentages: N/A
Computed value: computed color
Animation type: by computed value
Name: border-color
Value: <>{1,4}
Initial: see individual properties
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation Type: see individual properties
animations/border-color-interpolation.html
border-image-displayed-with-transparent-border-color.html
color-mix-currentcolor-border-repaint-parent.html
color-mix-currentcolor-border-repaint.html
currentcolor-border-repaint-parent.html
inheritance.sub.html
parsing/border-color-computed.html
parsing/border-color-invalid.html
parsing/border-color-shorthand.html
parsing/border-color-valid.html
These properties set the foreground color
of the [=border=] specified by the 'border-style' properties.
The 'border-color' property is a [=shorthand property=] for setting
'border-top-color', 'border-right-color', 'border-bottom-color', and 'border-left-color'
in a single declaration.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
Line Patterns: the 'border-style' properties
Name: border-top-style, border-right-style, border-bottom-style, border-left-style
Value: <>
Initial: none
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Logical property group: border-style
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
Name: border-style
Value: <>{1,4}
Initial: see individual properties
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation type: see individual properties
inheritance.sub.html
parsing/border-style-computed.html
parsing/border-style-invalid.html
parsing/border-style-shorthand.html
parsing/border-style-valid.html
These properties control whether a [=border=] appears,
and if it does what style it's drawn in
(if it is not overridden by a border image).
The 'border-style' property is a [=shorthand property=] for setting
'border-top-style', 'border-right-style', 'border-bottom-style', and 'border-left-style'
in a single declaration.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
The style is specified as a <> keyword, where
No border.
Color and width are ignored (i.e., the border has width 0).
Note this means that the initial value of 'border-image-width'
will also resolve to zero.
hidden
Same as ''border-style/none'',
but has different behavior in the border conflict resolution rules
for border-collapsed tables [[!CSS2]].
dotted
A series of round dots.
dashed
A series of square-ended dashes.
solid
A single line segment.
double
Two parallel solid lines with some space between them.
(The thickness of the lines is not specified,
but the sum of the lines and the space must equal 'border-width'.)
groove
Looks as if it were carved in the canvas.
(This is typically achieved by creating a “shadow” from two colors
that are slightly lighter and darker than the specified 'border-color'.)
ridge
Looks as if it were coming out of the canvas.
inset
Looks as if the content on the inside of the border
is sunken into the canvas.
Treated as ''ridge'' in the
collapsing border model. [[!CSS2]]
outset
Looks as if the content on the inside of the border
is raised out of the canvas.
Treated as ''groove'' in the
collapsing border model. [[!CSS2]]
Borders are drawn in front of the element's background,
but behind the element's content (in case it overlaps).
Example renderings of the predefined border styles.
Note: Border colors close to black or white
may need different color calculations than colors in between
in order to create the required “3D” effect
of ''groove'', ''ridge'', ''border-style/inset'', or ''border-style/outset''.
Note: There is no control over
the spacing of the dots and dashes,
nor over the length of the dashes.
Implementations are encouraged to choose a spacing
that makes the corners symmetrical.
Note: This specification does not define how borders of different styles
should be joined in the corner.
Also note that rounded corners may cause the corners and the contents to overlap,
if the padding is less than the radius of the corner.
Line Thickness: the 'border-width' properties
Name: border-top-width, border-right-width, border-bottom-width, border-left-width
Value: <>
Initial: medium
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Logical property group: border-width
Percentages: N/A
Computed value: absolute length, [=snapped as a border width=]
Animation Type: by computed value
Name: border-width
Value: <>{1,4}
Initial: see individual properties
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation Type: see individual properties
animations/border-bottom-width-composition.html
animations/border-image-width-composition.html
animations/border-image-width-interpolation.html
animations/border-left-width-composition.html
animations/border-right-width-composition.html
animations/border-top-width-composition.html
animations/border-width-interpolation.html
border-bottom-width-medium.html
border-bottom-width-thick.html
border-bottom-width-thin.html
border-left-width-medium.html
border-left-width-thick.html
border-left-width-thin.html
border-right-width-medium.html
border-right-width-thick.html
border-right-width-thin.html
border-top-width-medium.html
border-top-width-thick.html
border-top-width-thin.html
border-width-cssom.html
border-width-pixel-snapping-001-a.html
border-width-pixel-snapping-001-b.html
border-width-small-values-001-a.html
border-width-small-values-001-b.html
border-width-small-values-001-c.html
border-width-small-values-001-d.html
border-width-small-values-001-e.html
inheritance.sub.html
parsing/border-width-computed.html
parsing/border-width-invalid.html
parsing/border-width-shorthand.html
parsing/border-width-valid.html
These properties specify the thickness of the [=border=],
i.e. the border width.
Where
<> = <> | thin | medium | thick
Negative values are invalid.
The thin, medium, and thick keywords
are equivalent to ''1px'', ''3px'', and ''5px'', respectively.
The 'border-width' property is a [=shorthand property=] for setting
'border-top-width', 'border-right-width', 'border-bottom-width', and 'border-left-width'
in a single declaration.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
The [=resolved value=] for the 'border-width' properties is the [=used value=].
If the 'border-style' corresponding to a given 'border-width' is ''border-style/none'' or ''hidden'',
then the [=used value|used=] width is 0.
Note: Although the [=initial value|initial=] width is ''medium'',
the [=initial value|initial=] style is ''border-style/none'';
therefore the [=used value|used=] initial width is 0.
Border Shorthand Properties
Name: border-top, border-right, border-bottom, border-left
Value: <> || <> || <>
Initial: See individual properties
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation Type: see individual properties
These [=shorthand properties=] set
the 'border-width', 'border-color', and 'border-style'
of the top, right, bottom, and left [=borders=] of a box.
Omitted values are set to their [=initial values=].
Name: border
Value: <> || <> || <>
Initial: See individual properties
Applies to: all elements except [=ruby base containers=] and [=ruby annotation containers=]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation Type: see individual properties
parsing/border-invalid.html
parsing/border-shorthand.html
parsing/border-valid.html
The 'border' property is a [=shorthand property=] for setting
the same 'border-width', 'border-color', and 'border-style'
for all four borders of a box.
Unlike the shorthand 'margin' and 'padding' properties,
the 'border' property cannot set different values on the four borders.
To do so, one or more of the other border properties must be used.
The 'border' shorthand also resets 'border-image' to its initial value.
It is therefore recommended that authors use the 'border' shorthand,
rather than other shorthands or the individual properties,
to override any border settings earlier in the cascade.
This will ensure that 'border-image' has also been reset
to allow the new styles to take effect.
Note: The CSS Working Group intends for the 'border' shorthand
to reset all border properties in future levels of CSS as well.
For example, if a border-characters property
is introduced in the future to allow glyphs as borders,
it will also be reset by the 'border' shorthand.
By using the 'border' shorthand to reset borders,
authors can be guaranteed a “blank canvas”
no matter what properties are introduced in the future.
For example, the first rule below is equivalent to the set of five
rules shown after it:
p { border: solid red }
p {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red;
border-image: none;
}
Since, to some extent, the properties have overlapping functionality,
the order in which the rules are specified is important.
Consider this example:
blockquote {
border-color: red;
border-left: double;
color: black
}
In the above example,
the color of the left border is black, while the other borders are red.
This is due to 'border-left' setting the width, style, and color.
Since the color value is not given by the 'border-left' property,
it will be taken from the 'color' property.
The fact that the 'color' property is set after the 'border-left' property
is not relevant.
Rounded Corners
Curve Radii: the 'border-radius' properties
Name: border-top-left-radius, border-top-right-radius, border-bottom-right-radius, border-bottom-left-radius
Value: <>{1,2}
Initial: 0
Applies to: all elements (but see prose)
Inherited: no
Logical property group: border-radius
Percentages: Refer to corresponding dimension of the border box.
Computed value: pair of computed <> values
Animation Type: by computed value
Name: border-radius
Value: <>{1,4} [ / <>{1,4} ]?
Initial: see individual properties
Applies to: all elements (but see prose)
Inherited: no
Percentages: Refer to corresponding dimension of the border box.
Computed value: see individual properties
Animation Type: see individual properties
animations/border-bottom-left-radius-composition.html
animations/border-bottom-right-radius-composition.html
animations/border-radius-interpolation.html
animations/border-top-left-radius-composition.html
animations/border-top-right-radius-composition.html
border-bottom-left-radius-001.xht
border-bottom-left-radius-004.xht
border-bottom-left-radius-005.xht
border-bottom-left-radius-010.xht
border-bottom-left-radius-011.xht
border-bottom-left-radius-014.xht
border-bottom-right-radius-001.xht
border-bottom-right-radius-004.xht
border-bottom-right-radius-005.xht
border-bottom-right-radius-010.xht
border-bottom-right-radius-011.xht
border-bottom-right-radius-014.xht
border-radius-001.xht
border-radius-002.xht
border-radius-003.xht
border-radius-004.xht
border-radius-005.xht
border-radius-006.xht
border-radius-007.xht
border-radius-008.xht
border-radius-009.xht
border-radius-010.xht
border-radius-011.xht
border-radius-012.html
border-radius-013.html
border-radius-clip-001.html
border-radius-clip-002.htm
border-radius-clipping-002.html
border-radius-clipping-with-transform-001.html
border-radius-css-text.html
border-radius-dynamic-from-no-radius.html
border-radius-horizontal-value-is-zero.html
border-radius-shorthand-002.html
border-top-left-radius-001.xht
border-top-left-radius-004.xht
border-top-left-radius-005.xht
border-top-left-radius-010.xht
border-top-left-radius-011.xht
border-top-left-radius-014.xht
border-top-right-radius-001.xht
border-top-right-radius-004.xht
border-top-right-radius-005.xht
border-top-right-radius-010.xht
border-top-right-radius-011.xht
border-top-right-radius-014.xht
box-shadow-border-radius-001.html
css-border-radius-001.html
inheritance.sub.html
inner-border-non-renderable.html
parsing/border-radius-computed.html
parsing/border-radius-invalid.html
parsing/border-radius-valid.html
parsing/webkit-border-radius-valid.html
The two <> values of the 'border-*-radius' properties
define the radii of
a quarter ellipse that defines the shape of the corner
of the outer [=border edge=] (see the diagram below).
The first value is the horizontal radius,
the second the vertical radius.
If the second value is omitted it is copied from the first.
If either length is zero, the corner is square, not rounded.
Percentages for the horizontal radius refer to the width of the [=border box=],
whereas percentages for the vertical radius refer to the height of the [=border box=].
Negative values are invalid.
The two values of ''border-top-left-radius: 55pt 25pt''
define the curvature of the corner.
This example draws ovals of 15em wide and 10em high:
The 'border-radius' shorthand sets all four 'border-*-radius' properties.
If values are given before and after the slash,
then the values before the slash set the horizontal radii
and the values after the slash set the vertical radii.
If there is no slash, then the values set the radii in both axes equally.
The four values for each radii are given in the order
top-left, top-right, bottom-right, bottom-left.
If bottom-left is omitted it is the same as top-right.
If bottom-right is omitted it is the same as top-left.
If top-right is omitted it is the same as top-left.
The [=padding edge=] (inner border) radius is
the outer border radius minus the corresponding border thickness.
In the case where this results in a negative value,
the inner radius is zero.
(In such cases the center of the border’s inner curve might not coincide
with that of its outer curve.)
Likewise the [=content edge=] radius is
the [=padding edge=] radius minus the corresponding [=padding=],
or if that is negative, zero.
The border and padding thicknesses in the curved region
are thus interpolated from the adjoining sides,
and when two adjoining borders are of different thicknesses
the corner will show a smooth transition
between the thicker and thinner borders.
All border styles (''solid'', ''dotted'', ''border-style/inset'', etc.)
follow the curve of the border.
The effect of a rounded corner when the two borders it connects
are of unequal thickness (left)
and the effect of a rounded corner on borders
that are thicker than the radius of the corner (right).
Note: If the center of a corner's outer curve
is past an opposite [=padding edge=]
(in the [=border area=] of a side opposite the corner),
the inner curve will not be a full quarter ellipse.
Where the border-radius curve extends into the opposite sides' borders,
the arc of the padding edge is less than 90°.
The [=margin edge=], being outside the [=border edge=],
calculates its radius by adding the corresponding margin thickness
to each border radius, with the corresponding [=outset-adjusted border radius=] applied.
background-clip-content-box-with-border-radius-002.html
background-clip-content-box-with-border-radius-003.html
background-clip-padding-box-with-border-radius-002.html
background-clip-padding-box-with-border-radius-003.html
When expanding an [=edge=] that has a [=border radius=], e.g. for computing the [=margin edge=], 'box-shadow' spread, or 'overflow-clip-margin',
the different [=border radius=] values are adjusted so that a small rounded corner with a big outset does not appear to be disproportionally round.
This is done by computing the corresponding [=outset-adjusted border radius=].
To compute the outset-adjusted border radius given the 2-dimensional [=size=]'s |edge|, |radius|, and |outset|:
1. Let |coverage| be 2 * min(|radius|'s [=width=] / |edge|'s [=width=], |radius|'s [=height=] / |edge|'s [=height=]).
1. Let |adustedRadiusWidth| be the [=adjusted radius dimension=] given |coverage|, |radius|'s [=width=], and |outset|'s [=width=].
1. Let |adustedRadiusHeight| be the [=adjusted radius dimension=] given |coverage|, |radius|'s [=height=], and |outset|'s [=height=].
1. Return (|adustedRadiusWidth|, |adustedRadiusHeight|).
To compute the adjusted radius dimension given numbers |coverage|, |radius|, and |outset|:
1. If |radius| is greater than |spread|, or if |coverage| is greater than 1, then return |radius| + |outset|.
1. Let |ratio| be |radius| / |outset|.
1. Return |radius| + |outset| * (1 - (1 - |ratio|)3 * (1 - |coverage|3)).
Note: this algorithm is designed to reduce the effect of the |outset| (or spread) on the shape of the corner.
The |coverage| factor makes this reduction more pronounced for rectangular shapes (where the [=border radius=] is close to 0),
and less pronounced for elliptical shapes (where the [=border radius=] is close to 50%).
Corner Clipping
Although border images
are not affected by 'border-radius',
other effects that clip painting or event handling
to the [=border edge|border=], [=padding edge|padding=], or [=content edge|content=] edge
must clip to their respective curves.
For example,
backgrounds clip to the curve specified by 'background-clip',
'overflow' values other than ''visible'' to the curved [=padding edge=]
(when 'overflow' on both axes is not ''visible''),
[=replaced element=] content to the curved [=content edge=],
pointer events to the curved [=border edge=],
etc.
Note: As 'border-radius' reduces the interactive area of an element
authors should make sure the remaining interactive area conforms
to recommended minima for the platforms they target;
in particular, conforming to recommended minimum touch target sizes
may require larger widths and heights when 'border-radius' is used.
This example adds appropriate padding,
so that the contents do not overflow the corners.
Note that there is no border,
but the background will still have rounded corners.
Color and style transitions must be contained
within the segment of the border that intersects
the smallest rectangle that contains both border radii
as well as the center of the inner curve
(which may be a point representing the corner of the padding edge,
if the border radii are smaller than the [=border width=]).
If one of these borders is zero-width,
then the other border takes up the entire transitional area.
Otherwise,
the center of color and style transitions between adjoining borders
is a point along the curve that is a continuous monotonic function
of the ratio of the border widths.
However it is not defined what these transitions look like
or what function maps from this ratio to a point on the curve.
Given these corner shapes, color and style transitions
must be contained within the green region.
In case D the rectangle defined by the border radii
does not include the center of the inner curve (which is a sharp corner),
so the transition region is expanded to include that corner.
Transitions may take up the entire transition region,
but are not required to:
For example, a gradient color transition between two solid border styles
might take up only the region bounded by
the tips of the outer radii and the tips of the inner radii
(represented in case D by the dark green region).
Overlapping Curves
Corner curves must not overlap:
When the sum of any two adjacent border radii
exceeds the size of the border box,
UAs must proportionally reduce
the used values of all border radii
until none of them overlap.
The algorithm for reducing radii is as follows:
Let f = min(Li/Si),
where i ∈ {top, right, bottom, left},
Si is the sum of the two corresponding radii
of the corners on side i,
and Ltop = Lbottom = the width of the box,
and Lleft = Lright = the height of the box.
If f < 1, then all corner radii are reduced
by multiplying them by f.
Note: This formula ensures that quarter circles remain quarter circles
and large radii remain larger than smaller ones,
but it may reduce corners that were already small enough,
which may make borders of nearby elements that should look the same
look different.
If the curve interferes with UI elements such as scrollbars,
the UA may further reduce the used value of the affected border radii
(and only the affected border radii)
as much as necessary, but no more.
For example, the borders A of the figure below
might be the result of
all corners need to be reduced by a factor 0.8 to make them fit.
The used border radii thus are
0.4em (instead of 0.5em) and 1.6em (instead of 2em).
See borders B in the figure.
These rounded corner might be the result of
''width: 6em; height: 2.5em; border-radius: 0.5em 2em 0.5em 2em''
for A; and ditto but with ''height: 2em'' for B.
Effect on Tables
The 'border-radius' properties do apply to
''display/table'', ''inline-table'', and ''table-cell'' boxes
in separated borders mode (''border-collapse: separate'').
When 'border-collapse' is ''border-collapse/collapse'', they have no effect.
table-cell-background-local-002.html
table-cell-background-local-003.html
table-cell-background-local.html
ttwf-reftest-borderRadius.html
Border Images
Authors can specify an image to be used in place of the border styles.
In this case, the border's design is taken from the sides and corners
of an image specified with 'border-image-source',
whose pieces may be sliced, scaled, and stretched in various ways
to fit the size of the [=border image area=].
The border-image properties do not affect layout:
layout of the box, its content, and surrounding content
is based on the 'border-width' and 'border-style' properties only.
This example creates
a top and bottom border consisting of a whole number of orange diamonds
and a left and right border of a single, stretched diamond.
The corners are diamonds of a different color.
The image to tile is as follows.
Apart from the diamonds, it is transparent:
The image is 81 by 81 pixels and has to be divided into 9 equal parts.
The style rules could thus be as follows:
The result, when applied to a DIV of 12 by 5em,
will be similar to this:
This shows a more complicated example,
demonstrating how the border image corresponds to the fallback border-style
but can also extend beyond the border area.
The border image is a wavy green border with an extended corner effect:
The 'border-image-source' image,
with the four 'border-image-slice' cuts at 124px
dividing the image into nine parts.
The rest of the border properties then interact
to lay out the tiles as follows:
Diagram of all border-image properties and how they interact,
and showing the rendering with and without the border-image in effect.
Here, even though the 'border-width' is 12px,
the 'border-image-width' property computes to 124px.
The [=border image area=] is then outset 31px from the [=border box=]
and into the [=margin area=].
If the border-image fails to load
(or if border images are not supported by the UA),
the fallback rendering uses a green double border.
Notice that the 'border' shorthand resets 'border-image'.
This makes it easy to turn off or reset all border effects:
.notebox {
border: double orange;
/* must set 'border' shorthand first, otherwise it erases 'border-image' */
border-image: url("border.png") 30 round;
/* but other 'border' properties can be set after */
border-width: thin thick;
}
...
.sidebar .notebox {
box-shadow: 0 0 5px gray;
border-radius: 5px;
border: none; /* turn off all borders */
/* 'border' shorthand resets 'border-image' */
}
Name: border-image-source
Value: none | <>
Initial: none
Applies to: All elements, except internal table elements when 'border-collapse' is ''border-collapse/collapse''
Inherited: no
Percentages: N/A
Computed value: the keyword ''border-image-source/none'' or the computed <>
Animation type: discrete
animations/border-image-source-interpolation.html
css3-border-image-source.html
parsing/border-image-source-computed.sub.html
parsing/border-image-source-invalid.html
parsing/border-image-source-valid.html
Specifies an image to use as a border
in place of the rendering specified by the 'border-style' properties
and, if given the ''fill'' keyword in 'border-image-slice',
as an additional image backdrop for the element.
If the value is ''border-image-source/none'' or if the image cannot be displayed
(or the property doesn't apply), the border styles will be used;
otherwise the element's 'border-style' borders are not drawn
and this border image is drawn
as described in the sections below.
Image Slicing: the 'border-image-slice' property
Name: border-image-slice
Value: [<> | <>]{1,4} && fill?
Initial: 100%
Applies to: All elements, except internal table elements when 'border-collapse' is ''border-collapse/collapse''
Inherited: no
Percentages: refer to size of the border image
Computed value: four values, each either a number or percentage; plus a ''fill'' keyword if specified
Animation type: by computed value
animations/border-image-slice-composition.html
animations/border-image-slice-interpolation-stability.html
animations/border-image-slice-interpolation.html
border-image-slice-001.xht
border-image-slice-002.xht
border-image-slice-003.xht
border-image-slice-004.htm
border-image-slice-005.htm
border-image-slice-006.htm
border-image-slice-007.htm
border-image-slice-fill-001.html
border-image-slice-fill-002.html
border-image-slice-fill-003.html
border-image-slice-percentage.html
border-image-slice-shorthand-reset.html
parsing/border-image-slice-computed.html
parsing/border-image-slice-invalid.html
parsing/border-image-slice-valid.html
This property specifies inward offsets from
the top, right, bottom, and left edges of the image,
dividing it into nine regions: four corners, four edges and a middle.
The middle image part is discarded (treated as fully transparent)
unless the ''fill'' keyword is present.
(It is drawn over the background;
see Drawing the Border Image.)
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
<>
Percentages are relative to the size of the image:
the width of the image for the horizontal offsets,
the height for vertical offsets.
<>
Numbers represent pixels in the image
(if the image is a raster image)
or vector coordinates
(if the image is a vector image).
fill
The ''fill'' keyword, if present,
causes the middle part of the border-image to be preserved.
(By default it is discarded, i.e., treated as empty.)
Negative values are invalid.
Computed values larger than the size of the image are interpreted as ''100%''.
The regions given by the 'border-image-slice' values may overlap.
However if the sum of the right and left widths
is equal to or greater than the width of the image,
the images for the top and bottom edge and the middle part are empty--
which has the same effect as if a nonempty transparent image
had been specified for those parts.
Analogously for the top and bottom values.
If the image must be sized to determine the slices
(for example, for SVG images with no [=natural dimensions=]),
then it is sized using the default sizing algorithm
with no [=specified size=] and the [=border image area=] as the [=default object size=].
Diagram illustrating the cuts corresponding to the value ''25% 30% 12% 20%''
Drawing Areas: the 'border-image-width' property
Name: border-image-width
Value: [ <> | <> | auto ]{1,4}
Initial: 1
Applies to: All elements,
except internal table elements when 'border-collapse' is ''border-collapse/collapse''
Inherited: no
Percentages: Relative to width/height of the [=border image area=]
Computed value: four values,
each either a number,
the keyword ''border-image-width/auto'',
or a computed <> value
Animation type: by computed value
border-image-width-001.htm
border-image-width-005.xht
border-image-width-006.xht
border-image-width-007.xht
border-image-width-008.html
border-image-width-009.html
border-image-width-should-extend-to-padding.html
parsing/border-image-width-computed.html
parsing/border-image-width-invalid.html
parsing/border-image-width-valid.html
The [=border image=] is drawn inside an area called the border image area.
This is an area whose boundaries by default correspond to the [=border box=],
see 'border-image-outset'.
The four values of 'border-image-width' specify offsets
that are used to divide the [=border image area=] into nine
regions.
The offsets represent inward distances from
the top, right, bottom, and left sides of the area, respectively.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
Values have the following meanings:
<>
Percentages refer to the size of the [=border image area=]:
the width of the area for horizontal offsets,
the height for vertical offsets.
<>
Numbers represent multiples of the corresponding computed
border-width.
auto
If ''border-image-width/auto'' is specified
then the used 'border-image-width' is
the [=natural size|natural=] width or height (whichever is applicable)
of the corresponding image slice (see 'border-image-slice').
If the image does not have the required [=natural dimension=]
then the corresponding computed 'border-width' is used instead.
Negative values are invalid for any 'border-image-width' values.
If two opposite 'border-image-width' offsets are large enough that they overlap,
then the [=used values=] of all 'border-image-width' offsets
are proportionally reduced until they no longer overlap.
In mathematical notation:
Given Lwidth
as the width of the [=border image area=],
Lheight
as its height,
and Wside
as the 'border-image-width' offset for the side side,
let f = min(Lwidth/(Wleft+Wright),
Lheight/(Wtop+Wbottom)).
If f < 1,
then all W are reduced by multiplying them by f.
Edge Overhang: the 'border-image-outset' property
Name: border-image-outset
Value: [ <> | <> ]{1,4}
Initial: 0
Applies to: All elements, except internal table elements when 'border-collapse' is ''border-collapse/collapse''
Inherited: no
Percentages: N/A
Computed value: four values, each a number or absolute length
Animation type: by computed value
animations/border-image-outset-composition.html
animations/border-image-outset-interpolation.html
border-image-outset-003.html
parsing/border-image-outset-computed.html
parsing/border-image-outset-invalid.html
parsing/border-image-outset-valid.html
The values specify the amount by which the [=border image area=]
extends beyond the [=border box=].
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
<>
Represents an outset of the specified length.
<>
Represents an outset of the specified multiple of
the corresponding computed border-width.
Negative values are invalid.
Portions of the border-image that are rendered outside the [=border box=]
are [=ink overflow=] and
do not trigger scrolling.
Also such portions are invisible to mouse events
and do not capture such events on behalf of the element.
Note: Even though they never cause a scrolling mechanism,
outset images may still be clipped by an ancestor or by the viewport.
Image Tiling: the 'border-image-repeat' property
Name: border-image-repeat
Value: [ stretch | repeat | round | space ]{1,2}
Initial: stretch
Applies to: All elements, except internal table elements when 'border-collapse' is ''border-collapse/collapse''
Inherited: no
Percentages: N/A
Computed value: two keywords, one per axis
Animation type: discrete
animations/discrete-no-interpolation.html
border-image-repeat-002.htm
border-image-repeat-004.htm
border-image-repeat-005.html
border-image-repeat-1.html
border-image-repeat-repeat-001.html
border-image-repeat-round-003.html
border-image-repeat-round-1.html
border-image-repeat-round-2.html
border-image-repeat-round-stretch-001.html
border-image-repeat-round.html
border-image-repeat-space-011.html
border-image-repeat-space-1.html
border-image-repeat-space-10.html
border-image-repeat-space-2.html
border-image-repeat-space-3.html
border-image-repeat-space-4-ref-1.html
border-image-repeat-space-4.html
border-image-repeat-space-5-ref-1.html
border-image-repeat-space-5.html
border-image-repeat-space-6.html
border-image-repeat-space-7.html
border-image-repeat-space-8.html
border-image-repeat-space-9.html
border-image-repeat-stretch-round-001.html
border-image-repeat_repeatnegx_none_50px.html
css3-border-image-repeat-repeat.html
css3-border-image-repeat-stretch.html
parsing/border-image-repeat-computed.html
parsing/border-image-repeat-invalid.html
parsing/border-image-repeat-valid.html
This property specifies how the images for the sides
and the middle part of the [=border image=]
are scaled and tiled.
The first keyword applies to the horizontal scaling and tiling
of the top, middle and bottom parts,
the second to the vertical scaling and tiling
of the left, middle and right parts;
see Drawing the Border Image.
If the second keyword is absent, it is assumed to be the same as the first.
Values have the following meanings:
stretch
The image is stretched to fill its corresponding [=region=].
repeat
The image is tiled (repeated) to fill its corresponding [=region=].
round
The image is tiled (repeated) to fill its corresponding [=region=].
If it does not fill the area with a whole number of tiles,
the image is rescaled so that it does.
space
The image is tiled (repeated) to fill its corresponding [=region=].
If it does not fill the region with a whole number of tiles,
the extra space is distributed around the tiles.
The exact process for scaling and tiling the border-image parts
is given in the section below.
Drawing the Border Image
After the [=border image=] given by 'border-image-source'
is sliced by the 'border-image-slice' values,
the resulting nine images are scaled, positioned, and tiled
into their corresponding [=border image regions=] in four steps:
Scale to 'border-image-width'.
The two images for the top and bottom edges are made as tall
as the top and bottom [=border image regions=], respectively,
and their width is scaled proportionally.
The images for the left and right edges are made as wide
as the left and right [=border image regions=], respectively,
and their height is scaled proportionally.
The corner images are scaled to be as wide and as tall
as the their respective [=border image regions=].
The middle image's width is scaled by the same factor as the top image
unless that factor is zero or infinity,
in which case the scaling factor of the bottom is substituted,
and failing that, the width is not scaled.
The height of the middle image is scaled by the same factor as the left image
unless that factor is zero or infinity,
in which case the scaling factor of the right image is substituted,
and failing that, the height is not scaled.
Scale to 'border-image-repeat'.
If the first keyword is ''border-image-repeat/stretch'',
the top, middle and bottom images are further scaled
to be as wide as the middle region of the [=border image area=].
The height is not changed any further.
If the first keyword is ''border-image-repeat/round'',
the top, middle and bottom images are resized in width,
so that exactly a whole number of them fit
in the middle region of the [=border image area=],
exactly as for ''background-repeat/round'' in the 'background-repeat' property.
If the first keyword is ''border-image-repeat/repeat'' or ''border-image-repeat/space'',
the top, middle, and bottom images are not changed any further.
The effects of ''border-image-repeat/stretch'', ''border-image-repeat/round'', ''border-image-repeat/repeat'', and ''border-image-repeat/space''
for the second keyword are analogous,
acting on the height of the left, middle and right images.
Position the first tile.
If the first keyword is ''border-image-repeat/repeat'',
the top, middle, and bottom images
are centered horizontally in their respective regions.
Otherwise the images are placed at the left edge
of their respective regions of the [=border image area=].
If the second keyword is ''border-image-repeat/repeat'',
the left, middle, and right images
are centered vertically in their respective regions.
Otherwise the images are placed at the top edge
of their respective regions of the [=border image area=].
Tile and draw.
The images are then tiled to fill their respective regions.
In the case of ''border-image-repeat/space'',
any partial tiles are discarded and the extra space distributed
before, after, and between the tiles.
(I.e. the gap before the first tile,
the gap after the last tile,
and the gaps between tiles are equalized.)
This can result in empty border-image side regions.
The images are drawn at the same stacking level as normal borders:
immediately in front of the background layers.
The middle image is not drawn
unless ''fill'' was specified for 'border-image-source'.
Border Image Shorthand: the 'border-image' property
Name: border-image
Value: <<'border-image-source'>> || <<'border-image-slice'>> [ / <<'border-image-width'>> | / <<'border-image-width'>>? / <<'border-image-outset'>> ]? || <<'border-image-repeat'>>
Initial: See individual properties
Applies to: See individual properties
Inherited: no
Percentages: N/A
Computed value: See individual properties
Animation Type: See individual properties
border-image-002.html
border-image-003.html
border-image-004.html
border-image-006.html
border-image-007.html
border-image-011.html
border-image-012.html
border-image-013.html
border-image-017.xht
border-image-018.xht
border-image-019.xht
border-image-020.xht
border-image-calc.html
border-image-image-type-001.htm
border-image-image-type-002.htm
border-image-image-type-003.htm
border-image-image-type-004.htm
border-image-image-type-005.htm
border-image-round-and-stretch.html
border-image-shorthand-001.htm
border-image-shorthand-002.htm
border-image-shorthand-003.htm
border-image-space-001.html
parsing/border-image-invalid.html
parsing/border-image-shorthand.sub.html
parsing/border-image-valid.html
This is a shorthand property for setting
'border-image-source',
'border-image-slice',
'border-image-width',
'border-image-outset',
and 'border-image-repeat'
in a single declaration.
Omitted values are set to their [=initial values=].
Effect on Tables
The 'border-image' properties apply
to the border of tables and inline tables
that have 'border-collapse' set to ''border-collapse/collapse''.
However, this specification does not define
how such an image border is rendered.
In particular, it does not define how the image border interacts
with the borders of cells, rows and row groups at the edges of the table
(see border conflict resolution in [[!CSS2]]).
It is expected that a future specification will define the rendering.
It is recommended that UAs do not apply border images
to tables with collapsed borders until then.
Miscellaneous Effects
Note: The 'box-decoration-break' property,
which defines how backgrounds and borders apply to a fragmented box,
has been moved to the CSS Fragmentation Module. [[CSS-BREAK-3]]
Drop Shadows: the 'box-shadow' property
Name: box-shadow
Value: none | <>#
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: either the keyword ''box-shadow/none'' or a list,
each item consisting of four absolute lengths
plus a computed color and optionally also a ''box-shadow/inset'' keyword
Animation type: by computed value,
treating ''box-shadow/none'' as a zero-item list
and appending blank shadows (''transparent 0 0 0 0'')
with a corresponding ''box-shadow/inset'' keyword as needed
to match the longer list
if the shorter list is otherwise compatible with the longer one
animations/box-shadow-composition.html
animations/box-shadow-interpolation.html
box-shadow-005.html
box-shadow-029.html
box-shadow-039.html
box-shadow-040.html
box-shadow-041.html
box-shadow-042.html
box-shadow-body.html
box-shadow-border-radius-001.html
box-shadow-calc.html
box-shadow-currentcolor.html
box-shadow-inset-without-border-radius.html
box-shadow-invalid-001.html
box-shadow-multiple-001.html
box-shadow-outset-without-border-radius-001.html
box-shadow-overlapping-001.html
box-shadow-overlapping-002.html
box-shadow-overlapping-003.html
box-shadow-overlapping-004.html
box-shadow-radius-000.html
box-shadow-radius-001.html
box-shadow-table-border-collapse-001.html
box-shadow/slice-block-fragmentation-001.html
box-shadow/slice-block-fragmentation-002.html
box-shadow/slice-block-fragmentation-003.html
box-shadow/slice-inline-fragmentation-001.html
box-shadow/slice-inline-fragmentation-002.html
box-shadow/slice-inline-fragmentation-003.html
css-box-shadow-001.html
css3-box-shadow.html
fieldset-inset-shadow.html
inset-box-shadow-scroll.html
inset-box-shadow-stacking-context-scroll.html
inheritance.sub.html
parsing/box-shadow-computed.html
parsing/box-shadow-invalid.html
parsing/box-shadow-valid.html
The 'box-shadow' property attaches one or more drop shadows to the box.
The property accepts either
the none value,
which indicates no shadows,
or a comma-separated list of shadows, ordered front to back.
Each shadow is given as a <>,
represented by 2-4 length values, an optional color, and an optional ''box-shadow/inset'' keyword.
Omitted lengths are 0; omitted colors default to ''currentColor''.
<> = <>? && [ <>{2} [ <> <>? ]? ] && inset?
The components of each <> are interpreted as follows:
1st <>
Specifies the horizontal offset of the shadow.
A positive value draws a shadow that is offset to the right of the box,
a negative length to the left.
2nd <>
Specifies the vertical offset of the shadow.
A positive value offsets the shadow down, a negative one up.
3rd <>
Specifies the blur radius.
Negative values are invalid.
If the blur value is zero, the shadow’s edge is sharp.
Otherwise, the larger the value, the more the shadow’s edge is blurred.
See Shadow Blurring, below.
4th <>
Specifies the spread distance.
Positive values cause the shadow to expand in all directions by the specified radius.
Negative values cause the shadow to contract.
See Shadow Shape, below.
Note that for inner shadows,
expanding the shadow (creating more shadow area)
means contracting the shadow’s perimeter shape.
<>
Specifies the color of the shadow.
If the color is absent, it defaults to ''currentColor''.
inset
If present, the ''box-shadow/inset'' keyword changes the drop shadow
from an outer box-shadow
(one that shadows the box onto the canvas,
as if it were lifted above the canvas)
to an inner box-shadow
(one that shadows the canvas onto the box,
as if the box were cut out of the canvas and shifted behind it).
The example below demonstrates the effects of spread and blur on the shadow:
An [=outer box-shadow=] casts a shadow
as if the border-box of the element were opaque.
Assuming a spread distance of zero, its perimeter has
the exact same size and shape as the border box.
The shadow is drawn outside the border edge only:
it is clipped inside the border-box of the element.
An [=inner box-shadow=] casts a shadow
as if everything outside the padding edge were opaque.
Assuming a spread distance of zero, its perimeter has
the exact same size and shape as the padding box.
The shadow is drawn inside the padding edge only:
it is clipped outside the padding box of the element.
If a [=spread distance=] is defined,
the shadow perimeter defined above
is expanded outward (for [=outer box-shadows=])
or contracted inward (for [=inner box-shadows=])
by outsetting (insetting, for inner shadows)
the shadow's straight edges by the [=spread distance=]
(and flooring the resulting width/height at zero).
Below are some examples of an orange box with a blue border
being given a drop shadow.
To preserve the box's shape when spread is applied,
the corner radii of the shadow are also increased (decreased, for inner shadows)
from the border-box (padding-box) radii by adding (subtracting)
the [=spread distance=] (and flooring at zero).
For outer shadows, the [=border radius=] is then [=outset-adjusted border radius|adjusted=], independently in each dimension,
to preserve the sharpness of rounded corners.
The 'border-image' does not affect the shape of the box-shadow.
box-shadow-radius-generated.html
Blurring Shadow Edges
A non-zero [=blur radius=] indicates
that the resulting shadow should be blurred,
such as by a Gaussian filter.
The exact algorithm is not defined;
however the resulting shadow must approximate
(with each pixel being within 5% of its expected value)
the image that would be generated by applying to the shadow
a Gaussian blur with a standard deviation equal to half the blur radius.
Note: This means for a long, straight shadow edge,
the blur radius will create a visibly apparent color transition
approximately the twice length of the blur radius
that is perpendicular to and centered on the shadow's edge,
and that ranges
from almost the full shadow color at the endpoint inside the shadow
to almost fully transparent at the endpoint outside it.
Layering, Layout, and Other Details
The shadow effects are applied front-to-back:
the first shadow is on top and the others are layered behind.
Shadows do not influence layout and may overlap (or be overlapped by)
other boxes and text or their shadows.
In terms of stacking contexts and the painting order,
the [=outer box-shadows=] of an element are drawn immediately below the background of that element,
and the [=inner shadows=] of an element are drawn immediately above the background of that element
(below the borders and border image, if any).
Unless otherwise specified, drop shadows are only applied to the [=principal box=].
If the affected box has multiple fragments,
the shadows are applied as specified in 'box-decoration-break'.
Shadows are [=ink overflow=];
they do not trigger scrolling or increase the size of the [=scrollable overflow area=].
Outer shadows have no effect on internal table elements in the collapsing border model.
If a shadow is defined for single border edge in the collapsing border model
that has multiple border thicknesses
(e.g. an outer shadow on a table where one row has thicker borders than the others,
or an inner shadow on a rowspanning table cell that adjoins cells with different border thicknesses),
the exact position and rendering of its shadows are undefined
Levels
This section is informative.
CSS has different levels of features, each a subset of the other.
(See [[CSS-2017]] for a full explanation.)
The lists below describe which features from this specification are in each level.
Level 1
'background-color'
'background-image' only one image (no layers)
'background-repeat': only ''background-repeat/repeat'' | ''repeat-x'' | ''repeat-y'' | ''no-repeat''
'background-attachment': only ''scroll'' | ''fixed''
'background-position': only one or two values allowed
'background' shorthand: only color, image, repeat, attachment and position
'border-color' properties
'border-style' properties
'border-width' properties
'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' shorthands
Level 2
'background-color'
'background-image': only one image (no layers)
'background-repeat': only ''background-repeat/repeat'' | ''repeat-x'' | ''repeat-y'' | ''no-repeat''
'background-attachment': only ''scroll'' | ''fixed''
'background-position': only one or two values allowed
'background': only color, image, repeat, attachment and position
'border-color' properties
'border-style' properties
'border-width' properties
'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' shorthands
Level 3
All features described in the CSS Backgrounds and Borders Module Level 3
Changes
Changes since the 11 March 2024 Candidate Recommendation Draft
Changed the special case of 'border-width' having a value of ''0'' when 'border-style' is ''border-style/none'' or ''border-style/hidden''
from a computed-value effect to a resolved-value effect.
(Issue 11494)
Altered the border-radius outset adjustment,
so the radius reduction factor in the
minimum radius percentage of the corner, makes it less pronounced
the closer the corner's border-radius gets to 50%.
(Issue 7103)
Fixed syntax of 'box-shadow' to avoid allowing negative blur radii,
aligning it with the prose.
Cross linked to 'ink overflow' terminology
Changes since the 14 February 2023 Candidate Recommendation Snapshot
Defined serialization of 'background-position' in [[#bg-position-serialization]].
(Issue 2274)
Specified [=snapped as a border width|rounding of border widths=] to device pixels.
(Issue 7434)
Specified interaction of 'border-radius' and per-axis 'overflow' longhands.
(Issue 5210)
Fixed an error in the Computed Value line of 'background-image'.
(Issue 8604)
Removed the <box> definition
(replacing it with a <> reference),
as the Box Module now defines these terms.
Aligned property definition tables with the latest expectations
to include “Animation Type” and “Logical Property Group”.
Streamlined property grammar definitions using the latest
[=CSS/value definition syntax=].
Fixed the definition for where box shadows apply.
(Issue 9286)
Defined serialization of 'background-size' in [[#bg-size-serialization]].
(Issue 7802)
Changes since the 22 December 2020 Candidate Recommendation Snapshot
Clarified that the rule about not propagating backgrounds from the root
when it doesn't generate boxes only applies to ''display: none'',
not ''display: contents''.
(Issue 3779)
Precisely defined the size of 'border-width' ''thin'', ''medium'', and ''thick''.
(Issue 7254)
Minor editorial improvements.
Changes since the 17 October 2017 Candidate Recommendation
Inverted order of <> grammar to match browser serialization and 'text-shadow'/''drop-shadow()''.
(Issue 2305)
<> = inset<>? && <>{2,4} && <>inset?
Spread radius adjustment is only applied to shadows and margins
where the radius of curvature grows, rather than shrinks.
(Issue 1900)
To preserve the box's shape when spread is applied,
the corner radii of the shadow are also increased (decreased, for inner shadows)
from the border-box radii by adding (subtracting)
the [=spread distance=] (and flooring at zero).
However, in order to create a sharper corner when the border radius is small
(and thus ensure continuity between round and sharp corners),
when the [=border radius=] is less than the [=spread distance=]
(or in the case of an inner shadow,
less than the absolute value of a negative [=spread distance=]),
the [=spread distance=]
is multiplied by the proportion 1 + (r-1)3,
where r is the ratio of the border radius to the [=spread distance=],
in calculating the corner radii of the spread shadow shape.
Clarified that an omitted <> in a <> defaults to ''currentColor'',
not some mysterious unnamed value with the same behavior.
(2766)
If the color is absent, it defaults to ''currentColor''
the used color is taken from the 'color' property.
Cleaned up and regularized “Animation type” and “Computed value” lines
in the property definition tables.
Changed syntax to use the new CSS bracketed range notation
to reflect the prose restrictions on negative values,
and corrected a few grammar definition errors introduced in the previous publication.
Minor editorial improvements.
Changes since the 9 September 2014 Candidate Recommendation
Allow <<'background-clip'>> and <<'background-origin'>>
to be separated by other component values in the 'background' shorthand,
since this is what is implemented.
Allow <> and ''box-shadow/inset'' to be interleaved in any order in 'box-shadow',
since they are not ambiguous and CSS generally allows variant ordering where not ambiguous.
Define gradually increasing corner radius formula for 'box-shadow' spread curvature
to create continuity between sharp corners ('border-radius' = 0)
and curved corners ('border-radius' > spread distance).
This also gives better results for all intermediate states.
Add definition for how the margin edge is curved in response to border-radius.
(This is relevant for [[CSS-SHAPES]], but does not change conformance to CSS Backgrounds and Borders Level 3.)
Removed 'box-decoration-break'; it is now part of [[CSS-BREAK-3]].
Tighten up the definition of spread for 'box-shadow'.
Clarify that a 'border-style' of ''border-style/none'' also implies an initial 'border-image-width' of zero
(since 'border-image-width' is initially set to the computed 'border-width', which in this case is zero).
Clarified how ''background-attachment: local'' is affected by scrolling.
Simplified computed value of 'background-position'
to clarify that all 'background-position' values are interpolable.
Added “Animation Type” values to each property definition table.
Changes since the 17 April 2012 Candidate Recommendation
Section 2.1: Defined which properties
from this module apply to the ''::first-line'' and
''::first-letter'' pseudo-elements.
Section 3.4: Fixed the
incorrect definition of the computed value of
'background-repeat'. The value is always a pair of keywords, never a
single keyword.
Section 3.10: Added the missing
'background-image' to the list of properties that the 'background'
property reset.
Section 4.3: Added that negative
lengths are invalid on 'border-width'.
Section 5.1: Added that
negative lengths are invalid on 'border-radius'.
Section 6.8: Added a section
about the effect of 'border-image' on tables with collapsed borders
and added that the rendering will be defined later.
Changes Since the 15 February 2011 Candidate Recommendation
Section 3.9:
Defined what happens if 'background-size' has two ''background-size/auto'' values
and the image is missing a natural size.
If both values are ''background-size/auto'' then the natural width and/or
height of the image should be used, if any, the missing
dimension (if any) behaving as ‘auto’ as described above.
The center of color and style transitions between
adjoining borders is at the point on the curve that is at an angle
that is proportional to the ratio of the border widths. For
example, if the top and right border widths are equal, that point
is at a 45° angle from the horizontal, and if the top is twice the
width of the right the point is at a 30° angle from the
horizontal. The line demarcating this transition is drawn between
the point at that angle on the outer arc and the point at that
angle on the inner arc.
If one of these borders is zero-width, then the other
border takes up the entire transitional area. Otherwise, the
center of color and style transitions between adjoining borders
must be proportional to the ratio of the border widths such that a
function of its location is continuous with respect to this ratio.
However it is not defined what these transitions look like or how
“proportional” maps to a point on the curve.
If the image must be sized to determine the slices (for
example, for SVG images with no natural size), then it is sized
as for an auto-sized background, using the border image area as
the default object size in place of the background positioning
area.
Section 7.1
Optionally added bidi-imposed breaks to the types of breaks affected by
'box-decoration-break'.
Added:
UAs may also apply ‘box-decoration-break’ to control
rendering at bidi-imposed breaks, i.e. when bidi reordering causes
an inline to split into non-contiguous fragments. Otherwise such
breaks are always handled as ‘slice’.
Section 7.2:
Defined the default color of 'box-shadow'.
The color is the color of the shadow. If the color is
absent, the used color is taken from the ‘color’ property.
Section 3.6:
Clarified computed value of 'background-position'.
If one or two values are specified, for a <length> the
absolute length, otherwise a percentage. If three or four values
are specified, two pairs of a keyword plus a length or
percentage. Two keywords representing the origin and
two offsets from that origin, each given as an absolute length (if
given a <>), otherwise as a percentage.
Changed Equivalent to Computes in
definition of keywords.
Added "Animation Type" line to property definition tables.
Section 3.6:
Renamed <bg-position> production to <> for
easier re-use in other specifications and recast the grammar to be more
verbose but easier to understand.
bg-position<position> = [
[ left | center | right | top | bottom | <> ]
|
[ left | center | right | <> ]
[ top | center | bottom | <> ]?
|
[ center | [ left | right ] <>? ] &&
[ center | [ top | bottom ] <>? ]
]
Section 6.5:
Added ''border-image-repeat/space'' keyword to 'border-image-repeat' property value table:
it was included in the list of allowable values, but not in the syntax
definition.
[ stretch | repeat | round | space ]{1,2}
Section 5.5:
Corrected math error in border-radius example.
The height (2.5em) is enough for the specified radii (0.5em
plus 2.5em 2.0em).
Section 4.3:
Marked Percentages field as N/A for 'border-width', since they are not
included in the property.
Changes Since the 17 December 2009 Candidate Recommendation
Addition of ''background-clip/content-box'' value of 'background-clip'.
Change to the 'background' shorthand syntax for 'background-clip'
and 'background-origin'.
Removal of recommendation to use gradients for color transitions
when 'border-radius' produces a curve.
(Re)Addition of ''box-shadow'' property.
Various clarifications.
Acknowledgments
Tapas Roy was editor of the Border Module,
before it was merged with the Background Module.
Thanks to Ben Stucki for defining what happens with rounded corners
if the two adjoining borders are of unequal thickness or one of them is zero;
to Arjan Eising and Anne van Kesteren for the 'border-radius' syntax;
to Zack Weinberg for the corner transition regions diagram;
and to Lea Verou, plinss, and dbaron for the corner radius adjustment formula
(with special thanks to Lea for the live demo).
A set of properties for border images was initially proposed by fantasai.
The current simplification (one image cut into nine parts) is due to Ian Hickson.
(Though the original idea seems to originate with some anonymous Microsoft engineers.)
Finally, special thanks go to Brad Kemper for his feedback and suggestions
for many of the features in the draft,
for drawing all the box-shadow examples,
and for proposing some
radical changes
to the 'border-image' property
that solved a number of problems with the earlier definition.
Privacy Considerations
This specification introduces no new privacy considerations.
The image is 81 by 81 pixels and has to be divided into 9 equal parts.
The style rules could thus be as follows:
