CSS Backgrounds and Borders Module Level 4

Shortname: css-backgrounds
Level: 4
Status: ED
Work Status: Exploring
Group: csswg
TR: https://www.w3.org/TR/css-backgrounds-4/
ED: https://drafts.csswg.org/css-backgrounds-4/
Editor: Bert Bos, W3C, bert@w3.org
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
Editor: Lea Verou, Invited Expert, http://lea.verou.me/about/
Abstract: This module contains the features of CSS relating to the borders and backgrounds of boxes on the page. It includes and extends the functionality of CSS Backgrounds and Borders Level 3. [[CSS3BG]] The main extensions compared to level 3 are shaping ('corner-shape') and clipping borders ('border-clip'), logical background positions ('background-position'), and the ''extend'' ability of 'background-repeat'.
Ignored Terms:
Warning: Not Ready

Introduction

This module is currently maintained as a diff against Level 3. We will fold in the text once it's all formatted up and in CR again, as this will reduce the effort of keeping them in sync (source diffs will be accurate in reflecting the differences).

Backgrounds

Background Positioning: the 'background-position' shorthand property

		Name: background-position
		Value: <>#

See [[CSS3BG]] for definition.

Where

<position> = [
	  [ left | center | right | top | bottom | start | end | <> ]
	|
	  [ left | center | right | x-start | x-end | <> ]
	  [ top | center | bottom | y-start | y-end | <> ]
	|
	  [ center | [ left | right | x-start | x-end ] <>? ] &&
	  [ center | [ top | bottom | y-start | y-end ] <>? ]
	|
	  [ center | [ start | end ] <>? ]
	  [ center | [ start | end ] <>? ]	]

Values have the following meanings:

One value: If only one value is given, and that value is ''background-position/start'' or ''background-position/end'', then the keyword is duplicated; otherwise the second keyword defaults to ''background-position/center''. The resulting value is treated as a two-component value.
More than one value: If the value contains a ''background-position/start'' or ''background-position/end'' keyword, then the shorthand sets 'background-position-inline' and 'background-position-block' to the specified values. Otherwise the shorthand sets 'background-position-x' and 'background-position-y' to the specified values. Issue: Specify the value assignment in more detail. Should expand just like Level 3.

Issue: Specify what happens to set of properties that are not set. Maybe they're just not set?

Background Positioning Longhands: the 'background-position-x', 'background-position-y', 'background-position-inline', and 'background-position-block' properties

Issue: This section is still being worked out. The tricky thing is making all the start/end keywords work sanely.

		Name: background-position-x
		Value: [ center | [ left | right | x-start | x-end ]? <>? ]#
		Initial: left
		Inherited: no
		Percentages: refer to width of background positioning area minus width of background image
		Computed value: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword

This property specifies the background position's horizontal component. An omitted origin keyword is assumed to be ''left''.

		Name: background-position-y
		Value: [ center | [ top | bottom | y-start | y-end ]? <>? ]#
		Initial: left
		Inherited: no
		Percentages: refer to height of background positioning area minus height of background image
		Computed value: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword

This property specifies the background position's vertical component. An omitted origin keyword is assumed to be ''top''.

		Name: background-position-inline
		Value: [ center | [ start | end ]? <>? ]#
		Initial: not applicable (initial value comes from physical property)
		Inherited: no
		Percentages: refer to inline-size of background positioning area minus inline-size of background image
		Computed value: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword

This property specifies the background position's inline-axis component. An omitted origin keyword is assumed to be ''background-position-inline/start''.

		Name: background-position-block
		Value: [ center | [ start | end ]? <>? ]#
		Initial: not applicable (initial value comes from physical property)
		Inherited: no
		Percentages: refer to size of background positioning area minus size of background image
		Computed value: A list, each item consisting of: an offset given as a combination of an absolute length and a percentage, plus an origin keyword

This property specifies the background position's block-axis component. An omitted origin keyword is assumed to be ''background-position-block/start''.

Painting Area: the 'background-clip' property

	Name: background-clip
	New Values: text

Issue: Or should this be defining the -webkit-background-clip property, saying that all the values are identical, with this additional ''text'' value?

text: The background is painted within (clipped to) the union of the border box and the geometry of the text in the element and its in-flow and floated descendants.

Corners

Corner Sizing: the 'border-radius property

		Name: border-radius
		Value: <>{1,4} [ / <>{1,4} ]?
		Initial: 0
		Applies to: all elements, except table element when 'border-collapse' is ''collapse''
		Inherited: no

See [[CSS3BG]].

Corner Shaping: the 'corner-shape' property

		Name:            corner-shape
		Value:           [ round | bevel | scoop | notch ]{1,4}
		Initial:         round
		Applies to:      all elements, except table element when 'border-collapse' is ''collapse''
		Inherited:       no

By default, non-zero border-radii define a quarter-ellipse that rounds the affected corners. However in some cases, other corner shapes are desired. The 'corner-shape' property specifies a reinterpretation of the radii to define other corner shapes.

''round'': Border radii define a convex elliptical curve at the corner.
''bevel'': Border radii define a diagonal slice at the corner.
''scoop'': Border radii define a concave elliptical curve at the corner.
''notch'': Border radii define a concave rectangular notch at the corner.

For example, the following declarations create a right-pointing next button.

			a {
				border-radius: 0 2em 2em 0;
				corner-shape: bevel;
				padding: 0.5em 2em 0.5em 0.5em;

As a fallback in UAs that don't support 'border-radius', the right side would be rounded rather than pointy.

For ''scoop'' and ''notch'', how do the color/style/width transitions work? Do they transition once in the middle, or alternate to match the topness/sideness of that segment of the border?

Add a ''cubic-bezier()'' function for random other shapes? Or some other functions?

Corner Shape and Size: the 'corners' shorthand

		Name: corners
		Value: <<'corner-shape'>> || <<'border-radius'>>

The 'corners' shorthand sets 'corner-shape' and 'border-radius' in the same declaration. If either is omitted, it is reset to its initial value.

For example, the following declaration creates a diamond shape.

corners: bevel 50%;

In UAs that don't support 'corner-shape', the declaration is ignored (falls back to a rectangle).

In this example, the first declaration creates tabs with vertical sides and rounded corners using 'border-radius', while the second example makes them trapezoid-shaped in UAs that support 'corners'.

			border-radius: 0.25em 0.25em 0 0;
			corners: bevel 0.25em 0.25em 0 0 / 50% 50% 0 0;

Partial borders

CSS borders traditionally cover an entire border edge. Sometimes, however, it can be useful to hide some parts of the border.

Here are two proposals for doing this: the second one is from GCPM, the first one is an attempt to recast it more readably. The names are terrible, known problem, proposals accepted. There is a problem with conceiving this as clipping: if you have dotted borders, you want whole dots always, not parts of dots. So it should be a drawing limit, not a clip.

Partial Borders: the 'border-limit' property

  	Name: border-limit
  	Value: all | [ sides | corners ] <>?
  				| [ top | right | bottom | left ] <>
  	Initial: round
  	Applies to: all elements, except table element when 'border-collapse' is ''collapse''
  	Inherited: no
  	Percentages: relative to border-box

By default, the entire border is drawn. However, border rendering can be limited to only part of a border. The keyword specifies which part, and the length or percentage specifies how much.

''sides'': The sides are drawn up to but not including the corners (as defined by the border radii). A length or percentage is measured from the center of each side: ''50%'' draws the middle 50% of the border; by default the entire side is drawn.
''corners'': The corners are drawn plus the specified distance into the sides if specified. A length is measured from the closest edge of the corner area. A percentage is measured from the absolute corner of the border box.
''left''
''right'': For the left and right (vertical) sides, draws the entire side and corner. For the top and bottom (horizontal) sides, draws the left/right portion, as specified. Distances are measured as for ''corners''.
''top''
''bottom'': For the top and bottom (horizontal) sides, draws the entire side and corner. For the left and right (vertical) sides, draws the top/bottom portion, as specified. Distances are measured as for ''corners''.

The following example draws only the middle 50% of the sides.

box { border: solid; border-parts: sides 50% }

The following example draws only the curved parts of the corners.

box { border: solid; border-radius: 1em 2em; border-parts: corners; }

The following example draws only the left 4em of the top border.

box { border-top: solid; border-parts: left 4em; }
		The following example draws only the first 10px of each corner:
		box { border: solid; border-parts: corners 10px; }
		The following example draws the curved part of the corner plus 5px
		along ths sides:
		box { border: solid; border-radius: 5px; border-shape: scoop; border-parts: corners 5px; }
		The following example draws the curved part of the corner and all of
		the side except the middle 40%.
		box { border: solid; border-radius: 5px; border-shape: scoope; border-parts: corners 30%; }

The 'border-clip' properties

  		Name: border-clip, border-clip-top, border-clip-right, border-clip-bottom, border-clip-left
  		Value: normal | [ <> | <> ]+
  		Initial: normal
  		Inherited: no
  		Percentages: refer to length of border-edge side
  		Computed value: ''border-clip/normal'', or a list consisting of absolute lengths, or percentages as specified

Should these properties be simplified to only accept normal | <>+?

These properties split their respective borders into parts along the border edge. The first part is visible, the second is invisible, the third part is visible, etc. Parts can be specified with lengths, percentages, or flexible lengths (expressed by the ''fr'' unit, as per [[CSS3GRID]]). The ''border-clip/normal'' value means that the border is not split, but shown normally.

'border-clip' is a shorthand property for the four individual properties.

If the listed parts are shorter than the border, any remaining border is split proportionally between the specified flexible lengths. If there are no flexible lengths, the behavior is as if ''1fr'' had been specified at the end of the list.

If the listed parts are longer than the border, the specified parts will be shown in full until the end of the border. In this case, all flexible lengths will be zero.

For horizontal borders, parts are listed from left to right. For vertical borders, parts are listed from top to bottom.

The exact border parts are determined by laying out the specified border parts with all flexible lengths initially set to zero. Any remaining border is split proportionally between the flexible lengths specified.

border-clip: 10px 1fr 10px;

				border-clip-top: 10px 1fr 10px;
				border-clip-bottom: 10px 1fr 10px;
				border-clip-right: 5px 1fr 5px;
				border-clip-left: 5px 1fr 5px;

By making the first part have zero length, the inverse border of the previous example can easily be created:

				border-clip-top: 0 10px 1fr 10px;
				border-clip-bottom: 0 10px 1fr 10px;
				border-clip-right: 0 5px 1fr 5px;
				border-clip-left: 0 5px 1fr 5px;

				border: thin solid black;
				border-clip: 0 1fr; /* hide borders */
				border-clip-top: 10px 1fr 10px; /* make certain borders visible */
				border-clip-bottom: 10px 1fr 10px;

	border-top: thin solid black;
	border-bottom: thin solid black;
	border-clip-top: 10px;
	border-clip-bottom: 10px;

			border-top: thin solid black;
			border-clip: 10px;

This rendering:

				A sentence consists of words¹.

				¹ Most often.

can be achieved with this style sheet:

			@footnote {
				border-top: thin solid black;
				border-clip: 4em;
			}

			border: 2px solid black;
			border-top-parts: repeat(10px 10px);

In this example, the repeat pattern is shown five times and there is, by coincidence, no remaining border.

			border: 2px solid black;
			border-top-parts: repeat(10px 10px);

In this example, the repeat pattern is shown five times. The box in this example is slightly wider than the box in the previous example. The remaining border is taken up by a flexible length, as if this code had been specified:

			border: 2px solid black;
			border-top-parts: repeat(10px 10px) 1fr;

The fragment is shown in red for illustrative purposes; it should be shown in black by a compliant UA.

			border: 4px solid black;
			border-top-parts: 40px 20px 0 1fr repeat(20px 20px) 0 1fr 40px;

In this example, there will be a visible 40px border part on each end of the top border. Inside the 40px border parts, there will be an invisible border part of at least 20px. Inside these invisible border parts, there will be visible border parts, each 20px long with 20px invisible border parts between them.

The fragments are shown in red for illustrative purposes; they should not be visible in compliant UAs.

			border: 4px solid black;
			border-top-parts: 40px 20px 0 1fr 20px 20px 0 1fr 40px;

The fragments are shown in red for illustrative purposes; they should not be visible in compliant UAs.

			border: 4px solid black;
			border-clip-top: 3fr 10px 2fr 10px 1fr 10px 10px 10px 1fr 10px 2fr 10px 3fr;

All but one of the visible border parts are represented as flexible lengths in this example. The length of these border parts will change when the width of the element changes. Here is one rendering where 1fr ends up being 10px:

Here is another rendering where 1fr ends up being 30px:

The fragments are shown in red for illustrative purposes; they should be black in compliant UAs.

Changes

Additions Since Level 3

Additions are a work in progress... here's what we're planning to add. :)

logical 'background-position' values (''background-position/start'', ''background-position/end'')
the ''extend'' keyword of 'background-repeat'
'corner-shape'
multiple borders per element
logical border properties
Partial Borders (make part of border shorthand as well!)
More border-radius shorthands for doing both corners on a side at once.
Splitting horizontal / vertical spread radius for box-shadow, if we can come up with a sane syntax for it.

Acknowledgments

In addition to the many contributors to the [[CSS1]], [[CSS21]], and [[CSS3BG]] predecessors to this module, the editors would like to thank Tab Atkins, and Håkon Wium Lie for their suggestions and feedback specifically for this Level 4.