This section is not normative.
The CSS layout algorithms, by default,
size and position boxes in relation to each other
so that nothing overlaps.
This specification defines several ways to violate these assumptions when needed,
moving elements around in ways that can make them overlap other content:
* [=Relative positioning=], which visually shifts a box relative to its laid-out location.
* [=Sticky positioning=], which visually shifts a box relative to its laid-out location
in order to keep it visible when a scrollable ancestor
would otherwise scroll it out of sight.
* [=Absolute positioning=], which ignores normal layout entirely,
pulling the element [=out of flow=] and
positioning it relative to its [=containing block=]
with no regard for other content.
* [=Fixed positioning=], which absolutely positions the box
and affixes it to the viewport or page frame
so that it is always visible.
These [=positioning schemes=],
controlled by the 'position' property and the [=inset properties=],
are powerful
but easy to misuse.
With appropriate care,
they allow many interesting and useful layouts
that couldn't otherwise be achieved with standard layout rules;
without, they allow a page to be laid out in an unusable overlapping jumble of content.
Module Interactions
This module replaces and extends the [=positioning scheme=] features
defined in [[!CSS2]] sections:
It also replaces and supersedes the 'inset*' property definitions in [[CSS-LOGICAL-1]]
([[CSS-LOGICAL-1#inset-properties]]).
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.
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.
Choosing A Positioning Scheme: 'position' property
Name: position
Value: static | relative | absolute | sticky | fixed
Initial: static
Applies to: all elements except table-column-group and table-column
Inherited: no
Animatable: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
The 'position' property determines which of the positioning schemes
is used to calculate the position of a box.
Values other than ''static'' make the box a positioned box,
and cause it to establish an [=absolute positioning containing block=] for its descendants.
Values have the following meanings:
static
The box is not a [=positioned box=],
and is laid out according to the rules of its parent [=formatting context=].
The [=inset properties=] do not apply.
relative
The box is laid out as for ''static'',
then offset from the resulting position.
This offsetting is a purely visual effect,
and does not affect the size or position of any other box,
except insofar as it increases the [=scrollable overflow area=] of its ancestors.
This [=positioning scheme=] is called
relative positioning.
sticky
Identical to ''relative'',
except that its offsets are automatically adjusted
in reference to the nearest ancestor [=scroll container's=] [=scrollport=]
(as modified by the [=inset properties=])
in whichever axes the [=inset properties=] are not both ''top/auto'',
to try to keep the box in view within its [=containing block=]
as the user scrolls.
This [=positioning scheme=] is called
sticky positioning.
absolute
The box is taken [=out of flow=]
such that it has no impact on the size or position of its siblings and ancestors,
and does not participate in its parent’s [=formatting context=].
Instead, the box is positioned and sized solely in reference to
its [=absolute positioning containing block=],
as modified by the box's [=inset properties=],
see [[#abspos-layout]].
It can overlap [=in-flow=] content
or other [=absolutely positioned=] elements,
and is included in the [=scrollable overflow area=]
of the box that generates is [=containing block=].
This [=positioning scheme=] is called
absolute positioning.
fixed
Same as ''absolute'',
except the box is positioned and sized relative to a [=fixed positioning containing block=]
(usually the [=viewport=] in [=continuous media=], or the [=page area=] in [=paged media=]).
The box’s position is fixed with respect to this reference rectangle:
when attached to the [=viewport=]
it does not move when the document is scrolled,
and when attached to the [=page area=]
is replicated on every page when the document is paginated.
This [=positioning scheme=] is called
fixed positioning
and is considered a subset of [=absolute positioning=].
Authors may wish to specify ''fixed'' in a media-dependent way.
For instance, an author may want a box to remain at the top of the viewport on the screen,
but not at the top of each printed page.
The two specifications may be separated by using an '@media' rule, as in:
A 'position' value of ''position/absolute'' or ''position/fixed''
[=blockifies=] the box,
causes 'float' to compute to ''float/none'',
and forces the box to [=establish an independent formatting context=].
Containing Blocks of Positioned Boxes
The [=containing block=] of a ''position/static'', ''position/relative'', or ''position/sticky'' [=box=]
is as defined by its [=formatting context=].
For ''position/fixed'' and ''position/absolute'' boxes,
it is defined as follows:
If the box has ''position: absolute'':
The [=containing block=] is established
by the nearest ancestor box that establishes an absolute positioning containing block,
in the following way:
If the ancestor is not an [=inline box=],
the [=containing block=] is formed by the [=padding edge=] of the ancestor,
unless otherwise specified
(for example, see [[CSS-GRID-1#abspos-items]]).
If the ancestor is an [=inline box=],
the [=containing block=] is formed by
the [=block-start=] and [=inline-start=] [=content edges=] of the first [=box fragment=] of the ancestor,
and the [=block-end=] and [=inline-end=] [=content edges=] of the last [=box fragment=] of the ancestor.
Note: If the ancestor breaks across a line,
the “start” position might more end-ward than the “end” position.
If no ancestor establishes one,
the [=absolute positioning containing block=] is
the [=initial containing block=].
Note: Properties that can cause a box to establish
an [=absolute positioning containing block=] include
'position', 'transform', 'will-change', 'contain'…
If the box has ''position: fixed'':
The [=containing block=] is established
by the nearest ancestor box that establishes an fixed positioning containing block,
with the bounds of the [=containing block=]
determined identically to the [=absolute positioning containing block=].
Note: Properties that can cause a box to establish
a [=fixed positioning containing block=] include
'transform', 'will-change', 'contain'…
If no ancestor establishes one,
the [=fixed positioning containing block=] is:
* in [=continuous media=],
the viewport;
as a result, [=fixed=] boxes do not move when the document is scrolled.
Note: In this respect, they are similar to
[[css-backgrounds-3#background-attachment|fixed background images]]
(''background-attachment: fixed'').
* in [=paged media=],
the [=page area=] of each page;
[=fixed positioned=] [=boxes=] are thus replicated on every page.
(They are fixed with respect to the page box only,
and are not affected by being seen through a [=viewport=];
as in the case of print preview, for example.)
Note: As a result, parts of [=fixed-positioned boxes=]
that extend outside the [=initial containing block=]/[=page area=]
cannot be scrolled to
and will not print.
With no positioning, the containing blocks (C.B.) in the following document:
<!DOCTYPE html>
<html>
<head>
<title>Illustration of containing blocks</title>
</head>
<body id="body">
<div id="div1">
<p id="p1">This is text in the first paragraph...</p>
<p id="p2">This is text <em id="em1"> in the
<strong id="strong1">second</strong> paragraph.</em></p>
</div>
</body>
</html>
its containing block is no longer "body"; it becomes the initial containing block
(since there are no other positioned ancestor boxes).
If we position "em1" as well:
By positioning "em1", its containing block becomes the nearest positioned ancestor box
(i.e., that generated by "div1").
Positioning Coordinates
The precise location of a [=positioned box=] is controlled by
the inset properties:
the [=physical=] [=inset properties=] 'top', 'right', 'bottom', 'left';
the [=flow-relative=] [=inset properties=] 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end';
and their [=shorthands=], 'inset-block', 'inset-inline', and 'inset'.
The interpretation of these [=inset properties=] varies by [=positioning scheme=]:
* for [=absolute positioning=], they represent
insets from the containing block.
* for [=relative positioning=], they represent
insets from the box’s original margin edge.
* for [=sticky positioning=], they represent
insets from the [=scrollport=] edge.
Box Insets: the 'top', 'right', 'bottom', 'left', 'inset-block-start', 'inset-inline-start', 'inset-block-end', and 'inset-inline-end' properties
Name: top, right, bottom, left, inset-block-start, inset-inline-start, inset-block-end, inset-inline-end
Value: auto | <>
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: refer to size of containing block; see prose
Computed value: the keyword ''top/auto'' or a computed <> value
Animation type: by computed value type
These [=inset properties=] represent an inward “inset”
on the corresponding side of the box
(with respect to the box’s own [=writing mode=];
see [[CSS-WRITING-MODES-3#abstract-box]]).
For example, 'top' represents a downward inset of the top edge.
The [=physical=] and [=flow-relative=] properties
interact as defined in [[!CSS-LOGICAL-1]].
Values have the following meanings:
<length>
The inset is a fixed distance from the reference edge.
Negative values are allowed.
<percentage>
The inset is a percentage
relative to the containing block’s size
in the corresponding axis
(e.g. width for 'left' or 'right', height for 'top' and 'bottom').
For [=sticky positioned=] boxes,
the inset is instead
relative to the relevant [=scrollport=]’s size.
Negative values are allowed.
auto
Represents an unconstrained inset;
the exact meaning depends on the [=positioning scheme=].
Note: For [=fixed positioned=] elements,
using large values or negative values
can easily move elements outside the viewport
and make the contents unreachable through scrolling or other means.
Box Insets Shorthands: the 'inset-block', 'inset-inline', and 'inset' properties
Name: inset-block, inset-inline
Value: <'top'>{1,2}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
The 'inset-block' and 'inset-inline' properties are [=shorthand properties=]
for setting 'inset-block-start' + 'inset-block-end'
or 'inset-inline-start' + 'inset-inline-end',
respectively,
in a single declaration.
The first component value sets the [=CSS/start=] side,
the second sets the [=CSS/end=];
if omitted, the second value defaults to the first.
Name: inset
Value: <'top'>{1,4}
Initial: auto
Applies to: positioned elements
Inherited: no
Percentages: see individual properties
Computed value: see individual properties
Animation type: by computed value type
The 'inset' property is a [=shorthand property=]
that sets all of the [=inset properties=]
in a single declaration,
assigning values to the longhands representing each side
exactly as the 'margin' property does for its longhands.
By default, the 'inset' property values
are assigned to the corresponding [=physical=] [=longhand properties=]--
'top', 'right', 'bottom', and 'left'--
which for historical reasons do not have an inset- prefix.
This matches the behavior of other "4 values assigned to sides" properties,
such as 'margin'.
Allowing properties such as this to resolve to the [=flow-relative=] [=longhands=] is under discussion in [[CSS-LOGICAL-1]].
Yes, we understand it's a little confusing
that 'inset' doesn't expand to any 'inset-*' properties.
Relative Positioning
For a [=relatively positioned=] box,
the [=inset properties=] move the box inward
from the respective edge,
without changing its size.
'left' moves the box to the right, 'right' moves it to the left, etc.
Since boxes are not split or stretched as a result of [=relative positioning=]
opposing [=used values=] in a given axis must be negations of each other:
If opposing [=inset properties=] in an axis both compute to ''top/auto''
(their [=initial values=]),
their [=used values=] are zero
(i.e., the boxes stay in their original position in that axis).
If only one is ''top/auto'',
its [=used value=] becomes the negation of the other,
and the box is shifted by the specified amount.
If neither is ''top/auto'',
the position is over-constrained;
(with respect to the [=writing mode=] of its [=containing block=])
the [=computed value|computed=] [=CSS/end=] side value is ignored,
and its [=used value=] becomes the negation of the [=CSS/start=] side.
The following three rules are equivalent,
and shift the box ''1em'' to the left:
If specified on
a ''table-row-group'', ''table-header-group'', ''table-footer-group'', or ''table-row'' [=box=]
the shift affects all the contents of the box,
including all table cells that originate in the affected row,
but not those that don't.
Note: Since 'position' does not apply to ''table-column-group'' or ''table-column'' boxes,
they are not affected by [=relative positioning=].
Sticky positioning
[=Sticky positioning=] is similar to [=relative positioning=]
except the offsets are automatically calculated
in reference to the [=nearest scrollport=].
For a [=sticky positioned=] [=box=],
the [=inset properties=]
represent insets from the respective edges of the [=nearest scrollport=],
defining the sticky view rectangle
used to constrain the box’s position.
(For this purpose an ''top/auto'' value represents a zero inset.)
If this results in a [=sticky view rectangle=] size in any axis
less than the size of the [=border box=] of the [=sticky=] box in that axis,
then the effective [=end=]-edge inset in the affected axis is reduced
(possibly becoming negative)
to bring the [=sticky view rectangle=]’s size up to
the size of the [=border box=] in that axis
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
For example,
if the [=nearest scrollport=] is ''300px'' tall,
the [=sticky=] box's [=border box=] is ''200px'' tall,
and it has ''top: 20px'',
then the top-edge inset of the [=nearest scrollport=] is ''20px'',
and the bottom-edge inset is ''0px'',
yielding a [=sticky view rectangle=] that is ''280px'' tall.
But if the [=nearest scrollport=] were only ''100px'' tall,
then the effective bottom-edge inset becomes ''-120px'',
resulting in a [=sticky view rectangle=] that’s ''200px'' tall,
enough to fully contain the [=margin box=] of the [=sticky=] box.
For each side of the box,
if the corresponding [=inset property=] is not ''top/auto'',
and the corresponding [=border edge=] of the box would be outside
the corresponding edge of the [=sticky view rectangle=],
then the box must be visually shifted
([[#relpos-insets|as for relative positioning]])
to be inward of that [=sticky view rectangle=] edge,
insofar as it can while its [=position box=]
remains contained within its [=containing block=].
The position box is its [=margin box=],
except that for any side for which the distance between its [=margin edge=]
and the corresponding edge of its [=containing block=]
is less than its corresponding [=margin=],
that distance is used in place of that [=margin=].
Note: A sticky positioned element with
a non-''auto'' 'top' value and an ''auto'' 'bottom' value
will only ever be pushed down by sticky positioning;
it will never be offset upwards.
Note: Multiple [=sticky positioned=] boxes in the same container
are offset independently,
and therefore might overlap.
Scroll Position of Sticky-Positioned Boxes
For the purposes of any operation targeting the scroll position
of a sticky positioned element (or one of its descendants),
the sticky positioned element must be considered to be positioned
at its initial (non-offsetted) position.
For example, if a user clicks a link targeting a [=sticky-positioned=] element,
even if the element's [=nearest scrollport=] is currently scrolled
such that the [=sticky positioned=] element is offset from its initial position,
the [=scrollport=] will be scrolled back
so that the element's initial position is visible.
Absolute (and Fixed) Positioning
For an [=absolutely positioned=] box,
the [=inset properties=] effectively reduce the [=containing block=]
into which it is sized and positioned
by the specified amounts.
If only one [=inset property=] in a given axis is ''top/auto'',
it is set to zero.
If both [=inset properties=] in a given axis are ''top/auto'', then,
depending on the box’s [=self-alignment property=] in the relevant axis
(treating ''align-self/normal'' as ''align-self/start''
and any [=distributed alignment|distributed=], [=baseline alignment|baseline=], or ''align-self/stretch'' alignment value as its fallback alignment):
for ''align-self/self-start'' alignment or its equivalent
Set its start-edge [=inset property=] to the static position,
and its end-edge [=inset property=] to zero.
for ''align-self/self-end'' alignment or its equivalent
Set its end-edge [=inset property=] to the static position,
and its start-edge [=inset property=] to zero.
for ''align-self/center'' alignment
Let start distance be the distance from
the center of its [=static position rectangle=]
to the start edge of its [=containing block=],
and end distance be the distance from
the center of its [=static position rectangle=]
to the end edge of its [=containing block=].
If start distance is less than or equal to end distance,
then set the start-edge [=inset property=] to zero,
and set the end-edge [=inset property=] to
(containing block size - 2 × |start distance|);
otherwise,
set the end-edge [=inset property=] to zero
and the start-edge [=inset property=] to
(containing block size - 2 × |end distance|).
If these adjustments result in
an effective [=containing block=] size in any axis less than zero,
then the [=weaker inset=] in the affected axis
is reduced (possibly becoming negative)
to bring that size up to zero.
In the case that only one inset is ''auto'',
that is the weaker inset;
otherwise the [=weaker inset=] is the inset of the [=end=] edge
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
Note: Sizing and positioning of the [=absolutely positioned box=]
into this inset-modified containing block
is as described in [[#abspos-layout]].
If its [=self-alignment property=] in an axis is ''align-self/normal'',
then the [=resolved value=] of its [=weaker inset=] in that axis
is the value necessary to match that edge of its [=inset-modified containing block=]
to the corresponding edge of its [=margin box=]
after [[#abspos-layout|layout]].
(Otherwise the [=resolved value=] is the [=used value=] described above.)
Resolving Automatic Insets
When both [=inset properties=] in a given axis are ''left/auto'',
they are resolved in reference to the box’s static position rectangle,
whose edges represent the [=static position=] of the box
from each side of its [=containing block=].
This [=static position rectangle=]
and the [=static positions=] to which it corresponds
are defined by the layout model
the box would have had if its 'position' were ''position/static''
(and its 'float' and 'clear' values were ''float/none'').
(Note that this may require assuming a different hypothetical value
for 'display' as well.)
Block Layout
The [=static positions=] of a [=block-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static position rectangle=] is a zero-thickness rectangle spanning between
the inline-axis sides of the box’s [=static-position containing block=]
(see CSS2§10.3.7);
and positioned at its [=block-start=] [=static position=]
(see CSS2§10.6.4).
Inline Layout
The [=static positions=] of an [=inline-level box=]
are defined in [[CSS2]] Chapter 10.
The [=static position rectangle=] is a zero-thickness rectangle spanning between
the [=line-over=]/[=line-under=] sides of the [=line box=]
that would have contained its “hypothetical box”
(see CSS2§10.3.7);
and positioned at its [=inline-start=] [=static position=].
Flex Layout
The [=static position rectangle=] of the child of a [=flex container=]
corresponds to the [=content edges=] of the [=flex container=].
See static position of a grid container child in [[!CSS-FLEXBOX-1]].
Grid Layout
By default, the [=static position rectangle=] of the child of a [=grid container=]
corresponds to the [=content edges=] of the [=grid container=].
However, if that [=grid container=] also establishes
the box’s actual [=containing block=],
then the [=grid area=] specified by the [=grid-placement properties=]
establishes its [=static position rectangle=] instead.
See the static position of a grid container child in [[!CSS-GRID-1]].
For the purposes of calculating the [=static position rectangle=],
the [=containing block=] of [=fixed positioned=] elements
is the [=initial containing block=] instead of the [=viewport=],
and all [=scroll containers=] should be assumed
to be scrolled to their [=initial scroll position=].
Additionally, all ''margin/auto'' margins on the box itself
are treated as zero.
Fragmenting Absolutely-positioned Elements
In a [=fragmented flow=], an [=absolutely positioned box=]
is positioned relative to its [=containing block=]
ignoring any [=fragmentation breaks=]
(as if the flow were continuous).
The box may subsequently be broken over several [=fragmentation containers=].
For absolutely positioned content in [=paged media=]
that resolves to a position on a page
other than the page being laid out (the current page),
or resolves to a position on the current page
that has already been rendered for printing,
printers may place the content:
on the current page,
on a subsequent page, or
may omit it altogether.
Note: A [=block-level=] element that is split over several pages
can have a different width on each page,
and there may be device-specific limits.
User agents must not paginate the content of [=fixed-positioned boxes=].
Note: User agents might print invisible content in other ways.
See [[CSS-PAGE-3#content-outside-box]].
Absolute Positioning Layout Model
[=Absolute positioning=] not only takes a box [=out of flow=],
but also lays it out in its [=containing block=]
(after the final size of the [=containing block=] has been determined)
according to the absolute positioning layout model:
First, its [=inset-modified containing block=] is calculated,
defining its [=available space=].
(See [[#abspos-insets]].)
Because an [=absolutely positioned box=] can have no effect on the size of its [=containing block=],
its [=available space=] is always [=definite=].
Next, its width and height are resolved against this [=definite=] [=available space=],
as its [=preferred size=] capped by its [=maximum size=] (if any), floored by its [=minimum size=].
See [[#abspos-auto-size]].
Percentages, however,
are resolved against the original [=containing block=] size.
Then, the value of any ''margin/auto'' margins are calculated,
see [[#abspos-margins]].
Lastly, its [=margin box=] is aligned within the [=inset-modified containing block=]
as defined by its [=self-alignment properties=].
Automatic Sizes of Absolutely-Positioned Boxes
The [=automatic size=] of an [=absolutely positioned box=] is resolved against its [=inset-modified containing block=] as follows
(treating ''margin/auto'' [=margins=] as zero):
: If its [=self-alignment property=] in the relevant axis is ''align-self/stretch''
: Or if it is ''align-self/normal''
and the box is [=non-replaced=],
not a [=table wrapper box=],
and has no ''inset/auto'' [=inset property|inset=] in the relevant axis
::
Its [=automatic size=] is its [=stretch-fit size=].
: Otherwise
::
Its [=automatic size=] is its [=fit-content size=].
However, if the box has an aspect-ratio,
then an [=automatic size=] in the [=ratio-dependent axis=]
is instead resolved as a [=max-content size=].
When both axes have an [=automatic size=],
if only one axis has an ''inset/auto'' [=inset property|inset=]
then that axis is the [=ratio-dependent axis=],
else the [=block axis=] is the [=ratio-dependent axis=].
An [=automatic size=] in the [=ratio-determining axis=]
is determined as above.
The [=automatic minimum size=] of an absolutely-positioned box is always zero.
Auto Margins of Absolutely-Positioned Boxes
If either [=inset property=] in the relevant axis is ''top/auto'',
then any ''margin/auto'' [=margins=] resolve to zero.
Otherwise, the remaining space is calculated
as the size of its [=inset-modified containing block=] in the relevant axis
minus the box's used size in the relevant axis,
and this remaining space is divided among
any ''margin/auto'' margins in the relevant axis.
However,
(all with respect to the [=writing mode=] of the [=containing block=]),
if in the [=inline axis=]
the remaining space is negative
and both margins are ''margin/auto'',
the [=inline-start|start=] margin resolves to zero
and the [=inline-end|end=] margin receives the remaining space.
Old Absolute Positioning Layout Model
ISSUE: This section is being replaced with the new [[#abspos-layout]] section.
It is preserved here for comparison:
both models should yield the same result
in [=horizontal writing modes=] when the box’s [=self-alignment=] is ''align-self/normal''.
The Width of Absolutely-Positioned, Non-Replaced Elements
The constraint that determines the used values for these elements is:
'left' + 'margin-left' + 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + 'margin-right' +
'right' = width of containing block
If all three of 'left', 'width', and 'right' are ''left/auto'':
First set any ''margin/auto'' values for 'margin-left' and 'margin-right'
to ''0''.
Then, if the 'direction' property of
the element establishing the static-position containing block
is ''ltr'' set 'left' to the static position
and apply rule number three below;
otherwise, set 'right' to the static-position
and apply rule number one below.
If none of the three is ''left/auto'':
If both 'margin-left' and 'margin-right' are ''margin/auto'',
solve the equation under the extra constraint that
the two margins get equal values,
unless this would make them negative,
in which case when direction of the containing block is ''ltr'' (''rtl''),
set 'margin-left' ('margin-right') to ''0''
and solve for 'margin-right' ('margin-left').
If one of 'margin-left' or 'margin-right' is ''margin/auto'',
solve the equation for that value.
If the values are over-constrained,
ignore the value for 'left'
(in case the 'direction' property of the containing block is ''rtl'')
or 'right'
(in case 'direction' is ''ltr'') and solve for that value.
Otherwise,
set ''margin/auto'' values for 'margin-left' and 'margin-right' to ''0'',
and pick one of the following six rules that apply.
If 'left' and 'width' are ''left/auto'' and 'right' is not ''right/auto'',
then the width is shrink-to-fit.
Then solve for 'left'.
If 'left' and 'right' are ''left/auto'' and 'width' is not ''width/auto'',
then if the 'direction' property of
the element establishing the static-position containing block
is ''ltr''
set 'left' to the static-position,
otherwise set 'right' to the static-position.
Then solve for 'left' (if 'direction' is ''rtl'')
or 'right' (if 'direction' is ''ltr'').
If 'width' and 'right' are ''width/auto'' and 'left' is not ''left/auto'',
then the width is shrink-to-fit.
Then solve for 'right'.
If 'left' is ''left/auto'', 'width' and 'right' are not ''width/auto'',
then solve for 'left'.
If 'width' is ''width/auto'', 'left' and 'right' are not ''left/auto'',
then solve for 'width'.
If 'right' is ''right/auto'', 'left' and 'width' are not ''left/auto'',
then solve for 'right'.
Summary of rules for dir=ltr in horizontal writing modes
Is auto?
Result
'left'
'width'
'right'
'margin-left'
'margin-right'
✔
✔
✔
any
auto margins → zero
left → static pos
width → shrink-to-fit
right → solve
✘
✘
✘
✔
✘
auto margin → free space
✘
✔
✔
✔
margins split positive free space
right margin gets negative free space
✘
✘
treat 'right' as ''right/auto''
✔
✘
✔
any
auto margins → zero
left → static pos
width → as specified
right → solve
✔
✔
✘
any
auto margins → zero
left → solve
width → shrink-to-fit
right → as specified
✘
✔
✔
any
auto margins → zero
left → as specified
width → shrink-to-fit
right → solve
✔
✘
✘
any
auto margins → zero
solve for auto
✘
✘
✔
✘
✔
✘
The width of absolute or fixed positioned, replaced elements
If 'height' and 'width' both have computed values of auto
and the element also has an intrinsic width,
then that intrinsic width is the used value of 'width'.
If 'height' and 'width' both have computed values of auto
and the element has no intrinsic width,
but does have an intrinsic height and intrinsic ratio;
or if 'width' has a computed value of auto,
'height' has some other computed value,
and the element does have an intrinsic ratio;
then the used value of 'width' is:
(used height) * (intrinsic ratio)
If 'height' and 'width' both have computed values of auto,
the element has an intrinsic ratio but no intrinsic height or width,
and the containing block’s width
does not itself depend on the replaced element’s width,
then the used value of 'width' is calculated
from the constraint equation used for
block-level, non-replaced elements in normal flow.
Otherwise, if 'width' has a computed value of auto,
and the element has an intrinsic width,
then that intrinsic width is the used value of 'width'.
Otherwise, if 'width' has a computed value of auto,
but none of the conditions above are met,
and then the used value of 'width' becomes ''300px''.
If ''300px'' is too wide to fit the device,
user agents should use the width of the largest rectangle
that has a 2:1 ratio and fits the device instead.
After establishing the 'width',
in order to position the replaced element,
apply the following rules as appropriate.
If both 'left' and 'right' have the value auto,
and if the 'direction' property of
the element establishing the static-position containing block
is ''ltr'',
set 'left' to the static position and solve for 'right';
else if 'direction' is ''rtl'',
set 'right' to the static position and solve for 'left'.
If 'left' is auto and 'right' is not auto,
replace any auto on 'margin-left' or 'margin-right'
with ''0'',
then solve for 'left'.
If 'right' is auto and 'left' is not auto,
replace any auto on 'margin-left' or 'margin-right'
with ''0'',
then solve for 'right'.
If at this point both 'margin-left' and 'margin-right'
are still auto,
solve the equation under the extra constraint
that the two margins must get equal values,
unless this would make them negative,
in which case when the direction of the containing block
is ''ltr'' (''rtl''),
set 'margin-left' ('margin-right') to ''0''
and solve for 'margin-right' ('margin-left').
If at this point there is an auto remaining,
solve the equation for that value.
If at this point the values are over-constrained,
ignore the value for either 'left'
(in case the 'direction' property of the containing block is ''rtl'')
or 'right' (in case 'direction' is ''ltr'')
and solve for that value.
The Height Of Absolutely Positioned, Non-Replaced Elements
For absolutely positioned elements,
the used values of the vertical dimensions
must satisfy this constraint:
'top' + 'margin-top' + 'border-top-width' + 'padding-top' + 'height' +
'padding-bottom' + 'border-bottom-width' + 'margin-bottom' + 'bottom'
= height of containing block
If all three of 'top', 'height', and 'bottom' are ''top/auto'':
First set any ''top/auto'' values for 'margin-top' and 'margin-bottom' to ''0'',
then set 'top' to the static position,
and finally apply rule number three below.
If none of the three are ''top/auto'':
If both 'margin-top' and 'margin-bottom' are ''top/auto'',
solve the equation under the extra constraint that the two margins get equal values.
If one of 'margin-top' or 'margin-bottom' is ''top/auto'',
solve the equation for that value.
If the values are over-constrained,
ignore the value for 'bottom' and solve for that value.
Otherwise,
set ''top/auto'' values for 'margin-top' and 'margin-bottom' to ''0'',
and pick one of the following six rules that apply.
1. If 'top' and 'height' are ''top/auto''
and 'bottom' is not ''top/auto'',
then the height is based on the
Auto heights for block formatting context roots,
and solve for 'top'.
2. If 'top' and 'bottom' are ''top/auto''
and 'height' is not ''top/auto'',
then set 'top' to the static position,
then solve for 'bottom'.
3. If 'height' and 'bottom' are ''top/auto''
and 'top' is not ''top/auto'',
then the height is based on the
Auto heights for block formatting context roots,
and solve for 'bottom'.
4. If 'top' is ''top/auto'',
'height' and 'bottom' are not ''top/auto'',
then solve for 'top'.
5. If 'height' is ''top/auto'',
'top' and 'bottom' are not ''top/auto'',
then solve for 'height'.
6. If 'bottom' is ''top/auto'',
'top' and 'height' are not ''top/auto'',
then solve for 'bottom'.
The Height Of Absolutely Positioned, Replaced Elements
If 'height' and 'width' both have computed values of ''top/auto''
and the element also has an intrinsic height,
then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of ''top/auto''
and the element has an intrinsic ratio
then the used value of 'height' is:
(used width) / (intrinsic ratio)
Otherwise, if 'height' has a computed value of ''top/auto''
and the element has an intrinsic height,
then that intrinsic height is the used value of 'height'.
Otherwise, if 'height' has a computed value of ''top/auto'',
but none of the conditions above are met,
then the used value of 'height' must be set to
the height of the largest rectangle that has a 2:1 ratio,
has a height not greater than ''150px'',
and has a width not greater than the device width.
After establishing the 'height',
in order to position the replaced element,
apply the following rules as appropriate.
1. If both 'top' and 'bottom' have the value ''top/auto'',
replace 'top' with the element’s static position.
2. If 'bottom' is ''top/auto'',
replace any ''top/auto'' on 'margin-top' or 'margin-bottom'
with ''0''.
3. If at this point both 'margin-top' and 'margin-bottom' are still ''top/auto'',
solve the equation under the extra constraint
that the two margins must get equal values.
4. If at this point there is only one ''top/auto'' remaining,
solve the equation for that value.
5. If at this point the values are over-constrained,
ignore the value for 'bottom' and solve for that value.
Informative Comparison of Normal Flow, Floats, and Positioning
This section is not normative.
To illustrate the differences between normal flow,
[=relative positioning=], [=floats=], and [=absolute positioning=],
we provide a series of examples based on the following HTML:
<!DOCTYPE html>
<html>
<head>
<title>Comparison of positioning schemes</title>
<style>
body { display: block; font-size:12px; line-height: 200%;
width: 400px; height: 400px }
p { display: block }
span { display: inline }
</style>
</head>
<body>
<p>
Beginning of p contents.
<span id="outer"> Start of outer contents.
<span id="inner"> Inner contents.</span>
End of outer contents.</span>
End of p contents.
</p>
</body>
</html>
The final positions of boxes generated by the outer and inner
elements vary in each example. In each illustration, the numbers to the left of
the illustration indicate the normal flow position of the
double-spaced (for clarity) lines.
Note: The diagrams in this section are illustrative and not to scale. They are
meant to highlight the differences between the various positioning
schemes, and are not intended to be reference renderings of the
examples given.
Normal Flow Example
Consider the following CSS declarations for outer and inner
that do not alter the normal flow of boxes:
#outer { color: red }
#inner { color: blue }
The P element contains all inline content: anonymous inline text and
two SPAN elements. Therefore, all of the content will be laid out in an inline
formatting context, within a containing block established by the P element, producing
something like:
All of the text within the P's containing block flows together as continuous text,
even though it's located in separated nested elements.
#outer { position: relative; top: -12px; color: red }
#inner { position: relative; top: 12px; color: blue }
Text flows normally up to the outer element. The outer text is
then flowed into its normal flow position and dimensions at the end of line 1. Then,
the inline boxes containing the text (distributed over three lines) are shifted as a
unit by ''-12px'' (upwards).
The contents of inner, as a child of outer, would normally flow
immediately after the words "of outer contents" (on line 1.5). However, the inner
contents are themselves offset relative to the outer contents by ''12px''
(downwards), back to their original position on line 2.
Note that the content following outer is not affected by the relative
positioning of outer.
The result is identical to normal flow,
except that the "outer" text is shifted 12px upward,
without affecting the flow of the "body" or "inner" text.
Note also that had the offset of outer been ''-24px'', the text of outer
and the body text would have overlapped.
Floating Example
Now consider the effect of
floating the inner element’s text to the right by means of the
following rules:
#outer { color: red }
#inner { float: right; width: 130px; color: blue }
Text flows normally up to the inner box, which is pulled out of the flow
and floated to the right margin (its 'width' has been
assigned explicitly). Line boxes to the left of the float are shortened, and the
document’s remaining text flows into them.
The "inner" text lays out in an independent box on the right,
causing the remaining "body" and "outer" text to flow around it.
To show the effect of the 'clear' property, we add a sibling element to the
example:
<!DOCTYPE html>
<html>
<head>
<title>Comparison of positioning schemes II</title>
<style>
#inner { float: right; width: 130px; color: blue }
#sibling { color: red }
</style>
</head>
<body>
<p>
Beginning of p contents.
<span id="outer"> Start of outer contents.
<span id="inner"> Inner contents.</span>
<span id="sibling"> Sibling contents.</span>
End of outer contents.</span>
End of p contents.
</p>
</body>
</html>
These styles cause the inner box to float to the right, as before,
and the document’s remaining text to flow into the vacated space:
Identical to the previous example,
save that there is now "sibling" text
flowing with the "body" and "outer" text.
However, if the 'clear' property on the sibling element is set to 'right'
(i.e., the generated sibling box will not accept a position next to
floating boxes to its right), the sibling content begins to flow below the
float:
#inner { float: right; width: 130px; color: blue }
#sibling { clear: right; color: red }
Now the "sibling" text moves down to below the "inner" text’s box,
leaving blank space behind.
The text following the "sibling" text flows after it as normal.
Absolute Positioning Example
Next, we consider the effect of absolute positioning. Consider the following CSS
declarations for outer and inner:
which cause the top of the outer box to be positioned with respect to its
containing block. The containing block for a positioned box is established by the
nearest positioned ancestor (or, if none exists, the initial containing block, as in
our example). The top side of the outer box is ''200px'' below the top of the
containing block and the left side is ''200px'' from the left side. The child box of
outer is flowed normally with respect to its parent.
All of the text within #outer (the "outer" and "inner" text)
moves down to an independent box in the lower right corner.
The two halves of "body" text flow together.
The following example shows an absolutely positioned box that is a child of a relatively
positioned box. Although the parent outer box is not actually offset, setting
its 'position' property to ''position/relative'' means that its box may serve as the containing
block for positioned descendants. Since the outer box is an inline box that is
split across several lines, the first inline box’s top and left edges (depicted by thick
dashed lines in the illustration below) serve as references for 'top' and 'left' offsets.
This results in something like the following:
The "inner" text is positioned in an independent box,
relative to the top-left corner of the start of the "outer" text.
If we do not position the outer box:
the containing block for inner becomes the initial
containing block (in our example). The following illustration shows where
the inner box would end up in this case.
Same as before,
except now the "inner text" is positioned relative to the top-left corner of the page itself.
Relative and absolute positioning may be used to implement change bars, as shown
in the following example. The following fragment:
<p style="position: relative; margin-right: 10px; left: 10px;">
I used two red hyphens to serve as a change bar. They
will "float" to the left of the line containing THIS
<span style="position: absolute; top: auto; left: -1em; color: red;">--</span>
word.
</p>
might result in something like:
The two red hyphens, indicating a change,
sit in the left margin of the page
on the line containing the word "THIS",
regardless of what line that ends up being.
First, the paragraph (whose containing block sides are shown in the
illustration) is flowed normally. Then it is offset ''10px'' from the left edge
of the containing block (thus, a right margin of ''10px'' has been
reserved in anticipation of the offset). The two hyphens acting as change bars
are taken out of the flow and positioned at the current line (due to 'top: auto'),
''-1em'' from the left edge of its containing block (established by the P in
its final position). The result is that the change bars seem to "float" to the
left of the current line.
Acknowledgments
This module would not have been possible without input and support from many
helpful people. Thanks to
Rossen Atanassov,
Bert Bos,
Oriol Brufau,
Tantek Çelik,
Arron Eicholz
Sylvain Galineau,
John Jansen,
Chris Jones,
Ian Kilpatrick,
Anton Prowse.
Defined automatic sizing of non-replaced absolutely-positioned boxes with an aspect ratio.
(Issue 5151
However, if the box has an aspect-ratio,
then an [=automatic size=] in the [=ratio-dependent axis=]
is instead resolved as a [=max-content size=].
When both axes have an [=automatic size=],
if only one axis has an ''inset/auto'' inset
then that axis is the [=ratio-dependent axis=],
else the [=block axis=] is the [=ratio-dependent axis=].
An [=automatic size=] in the [=ratio-determining axis=]
is determined as above.
Aligned resolution of auto margins and automatic sizes of positioned boxes with [[CSS2]].
(Issue 5374,
Issue 5077,
Issue 5327)
If this results in a [=sticky view rectangle=] size in any axis
less than the size of the [=border box=] of the [=sticky=] box in that axis,
then the effective [=end=]-edge inset in the affected axis is reduced
(possibly becoming negative)
to bring the [=sticky view rectangle=]’s size up to
the size of the [=border box=] in that axis
(where [=end=] is interpreted relative to the [=writing mode=] of the [=containing block=]).
If neither is ''top/auto'',
the position is over-constrained;
(with respect to the [=writing mode=] of its [=containing block=])
the [=computed value|computed=] [=CSS/end=] side value is ignored,
and its [=used value=] becomes the negation of the [=CSS/start=] side.
However,
(all with respect to the [=writing mode=] of the [=containing block=]),
if in the [=inline axis=]
the remaining space is negative
and both margins are ''margin/auto'',
the [=inline-start|start=] margin resolves to zero
and the [=inline-end|end=] margin receives the remaining space.
: If its [=self-alignment property=] in the relevant axis is ''align-self/stretch''
: Or if it is ''align-self/normal''
and the box is [=non-replaced=] and has no ''inset/auto'' [=inset property|inset=] in the relevant axis
::
Its [=automatic size=] is its [=stretch-fit size=].
Rewrote the whole spec for editorial clarity, technical precision, and compatibility with [[CSS-ALIGN-3]], [[CSS-WRITING-MODES-3]], [[CSS-BREAK-3]], and [[CSS-DISPLAY-3]].
Privacy and Security Considerations
This specification introduces no new privacy considerations.
If an attacker is able to inject arbitrary CSS,
positioned layout can make it easier to position elements the attacker has control of
over arbitrary other elements of the page,
potentially tricking users of the page.
(There are many routes to this attack: negative 'margin', 'transform', etc.
Don't let people apply arbitrary CSS to bits of your page.)
''position: fixed'' can allow a page to emulate modal dialogs,
potentially tricking a user into thinking they're interacting with the user agent
and entering in sensitive information that they page can then capture.
User agents must ensure that their native dialogs are positioned
in ways that the page cannot emulate;
in particular,
that at least some of the dialog is outside the "poisoned pixels" that web content can paint to.
