CSS Borders and Box Decorations Module Level 4

Editor’s Draft,

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https://drafts.csswg.org/css-borders-4/
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(W3C)
Elika J. Etemad / fantasai (Apple)
Lea Verou (Invited Expert)
(Invited Expert)
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Abstract

This module contains the features of CSS relating to the borders and decorations of boxes on the page.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-borders” in the title, like this: “[css-borders] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 03 November 2023 W3C Process Document.

1. Introduction

This module is currently maintained as a diff against the parts related to borders and box decorations of CSS Backgrounds and Borders Module Level 3 [CSS3BG]. 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).

2. 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.

2.1. Line Colors: the border-color properties

Name: border-top-color, border-right-color, border-bottom-color, border-left-color, border-block-start-color, border-block-end-color, border-inline-start-color, border-inline-end-color
Value: <color> | <image-1D>
Initial: currentcolor
Applies to: all elements except ruby base containers and ruby annotation containers
Inherited: no
Percentages: N/A
Computed value: the computed color and/or a one-dimensional image function
Canonical order: per grammar
Animation type: see prose
Logical property group: border-color
Name: border-color
Value: [ <color> | <image-1D> ]{1,4}
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

These properties set the foreground color of the border specified by the border-style properties.

The stripes defined by <image-1D> follow the shape of the border on the side to which they apply, and are drawn in bands starting from the padding edge and progressing outwards. The border width at each point defines the total width of the stripes at that point.

Using multiple colors for each side:
.foo {
  border: 30px solid;
  border-color: stripes(dodgerblue, skyblue) stripes(yellow, gold) stripes(lightgreen, limegreen) stripes(indianred, orange);
}

Sample rendering:

The same border colors with border-style: dotted:

border-color is a shorthand for the four border-*-color properties. The four values set the top, right, bottom and left border, respectively. A missing left is the same as right, a missing bottom is the same as top, and a missing right is also the same as top. This is resolved individually for each list item.

The flow-relative properties border-block-start-color, border-block-end-color, border-inline-start-color, and border-inline-end-color correspond to the physical properties border-top-color, border-bottom-color, border-left-color, and border-right-color. The mapping depends on the element’s writing-mode, direction, and text-orientation.

Name: border-block-color, border-inline-color
Value: <'border-top-color'>{1,2}
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

These two shorthand properties set the border-block-start-color & border-block-end-color and border-inline-start-color & border-inline-end-color, respectively. The first value represents the start side color, and the second value represents the end side color. If only one value is given, it applies to both the start and end sides.

2.2. Line Patterns: the border-style properties

Name: border-top-style, border-right-style, border-bottom-style, border-left-style, border-block-start-style, border-block-end-style, border-inline-start-style, border-inline-end-style
Value: <line-style>
Initial: none
Applies to: all elements except ruby base containers and ruby annotation containers
Inherited: no
Percentages: N/A
Computed value: specified keyword
Canonical order: per grammar
Animation type: discrete
Logical property group: border-style

The flow-relative properties border-block-start-style, border-block-end-style, border-inline-start-style, and border-inline-end-style correspond to the physical properties border-top-style, border-bottom-style, border-left-style, and border-right-style. The mapping depends on the element’s writing-mode, direction, and text-orientation.

Name: border-block-style, border-inline-style
Value: <'border-top-style'>{1,2}
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

These two shorthand properties set the border-block-start-style & border-block-end-style and border-inline-start-style & border-inline-end-style, respectively. The first value represents the start side style, and the second value represents the end side style. If only one value is given, it applies to both the start and end sides.

2.3. Line Thickness: the border-width properties

Name: border-top-width, border-right-width, border-bottom-width, border-left-width, border-block-start-width, border-block-end-width, border-inline-start-width, border-inline-end-width
Value: <line-width>
Initial: medium
Applies to: all elements except ruby base containers and ruby annotation containers
Inherited: no
Percentages: N/A
Computed value: absolute length, snapped as a border width; zero if the border style is none or hidden
Canonical order: per grammar
Animation type: by computed value
Logical property group: border-width

The flow-relative properties border-block-start-width, border-block-end-width, border-inline-start-width, and border-inline-end-width correspond to the physical properties border-top-width, border-bottom-width, border-left-width, and border-right-width. The mapping depends on the element’s writing-mode, direction, and text-orientation.

Name: border-block-width, border-inline-width
Value: <'border-top-width'>{1,2}
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

These two shorthand properties set the border-block-start-width & border-block-end-width and border-inline-start-width & border-inline-end-width, respectively. The first value represents the start side width, and the second value represents the end side width. If only one value is given, it applies to both the start and end sides.

2.4. Border Shorthand Properties

Name: border-top, border-right, border-bottom, border-left, border-block-start, border-block-end, border-inline-start, border-inline-end
Value: <line-width> || <line-style> || <color>
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
Canonical order: per grammar

The flow-relative properties border-block-start, border-block-end, border-inline-start, and border-inline-end correspond to the physical properties border-top, border-bottom, border-left, and border-right. The mapping depends on the element’s writing-mode, direction, and text-orientation.

Name: border-block, border-inline
Value: <'border-block-start'>
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

These two shorthand properties set the border-block-start & border-block-end or border-inline-start & border-inline-end, respectively, both to the same style.

3. Corners

3.1. Corner Sizing: the border-*-*-radius properties

Name: border-top-left-radius, border-top-right-radius, border-bottom-right-radius, border-bottom-left-radius, border-start-start-radius, border-start-end-radius, border-end-start-radius, border-end-end-radius
Value: <length-percentage [0,∞]>{1,2}
Initial: 0
Applies to: all elements (but see prose)
Inherited: no
Percentages: Refer to corresponding dimension of the border box.
Computed value: pair of computed <length-percentage> values
Canonical order: per grammar
Animation type: by computed value
Logical property group: border-radius

The flow-relative properties border-start-start-radius, border-start-end-radius, border-end-start-radius, and border-end-end-radius correspond to the physical properties border-top-left-radius, border-bottom-left-radius, border-top-right-radius, and border-bottom-right-radius. The mapping depends on the element’s writing-mode, direction, and text-orientation, with the first start/end giving the block axis side, and the second the inline-axis side (i.e. patterned as 'border-block-inline-radius').

3.2. Corner Sizing Shorthands: the border-radius and border-*-radius shorthand properties

3.2.1. Sizing The Corners Of One Side: The border-top-radius, border-right-radius, border-bottom-radius, border-left-radius, border-block-start-radius, border-block-end-radius, border-inline-start-radius, border-inline-end-radius shorthands

Name: border-top-radius, border-right-radius, border-bottom-radius, border-left-radius, border-block-start-radius, border-block-end-radius, border-inline-start-radius, border-inline-end-radius
Value: <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]?
Initial: 0
Applies to: all elements (but see prose)
Inherited: no
Percentages: Refer to corresponding dimension of the border box.
Computed value: see individual properties
Canonical order: per grammar
Animation type: see individual properties

The border-*-radius shorthands set the two border-*-*-radius longhand properties of the related side. If values are given before and after the slash, then the values before the slash set the horizontal radius and the values after the slash set the vertical radius. If there is no slash, then the values set both radii equally. The two values for the radii are given in the order top-left, top-right for border-top-radius, top-right, bottom-right for border-right-radius, bottom-left, bottom-right for border-bottom-radius, top-left, bottom-left for border-left-radius, start-start, start-end for border-block-start-radius, end-start, end-end for border-block-end-radius start-start, end-start for border-inline-start-radius, and start-end, end-end for border-inline-end-radius. If the second value is omitted it is copied from the first.

3.2.2. Sizing All Corners At Once: The border-radius shorthand

Name: border-radius
Value: <length-percentage [0,∞]>{1,4} [ / <length-percentage [0,∞]>{1,4} ]?
Initial: 0
Applies to: all elements, except table element when border-collapse is collapse
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: see individual properties

See [CSS3BG].

3.3. Corner Shaping: the corner-shape property

Name: corner-shape
Value: [ round | angle ]{1,4}
Initial: round
Applies to: all elements, except table element when border-collapse is collapse
Inherited: no
Percentages: n/a
Computed value: as specified
Canonical order: per grammar
Animation type: discrete

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.
angle
Border radii define a diagonal slice at the corner.
For example, the following declarations create a right-pointing next button.
a {
  border-radius: .3em .8em .8em .3em / .3em 50% 50% .3em;
  corner-shape: round angle angle round;
  padding: .5em 1em .5em .5em;
}
Next
As a fallback in UAs that don’t support border-radius, the right side would be rounded rather than pointy:
Next

How to allow custom corners? Perhaps a path() function? Or a cubic-bezier()? Something else?

3.4. Corner Shape and Size: the corners shorthand

Name: corners
Value: <'corner-shape'> || <'border-radius'>
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

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: angle 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: angle 0.25em 0.25em 0 0 / 50% 50% 0 0;

4. 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.

4.1. Partial Borders: the border-limit property

Name: border-limit
Value: all | [ sides | corners ] <length-percentage [0,∞]>? | [ top | right | bottom | left ] <length-percentage [0,∞]>
Initial: all
Applies to: all elements, except table element when border-collapse is collapse
Inherited: no
Percentages: relative to border-box
Computed value: as specified
Canonical order: per grammar
Animation type: discrete

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.

all
The entire border is drawn.
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.

div {
  border: solid;
  border-limit: sides 50%;
}

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

div {
  border: solid;
  border-radius: 1em 2em;
  border-limit: corners;
}

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

div {
  border-top: solid;
  border-limit: left 4em;
}

The following example draws only the first 10px of each corner:

div {
  border: solid;
  border-limit: corners 10px;
}

The following example draws the curved part of the corner plus 5px along the sides:

div {
  border: solid;
  border-radius: 5px;
  border-limit: corners 5px;
}

The following example draws the curved part of the corner and all of the side except the middle 40%.

div {
  border: solid;
  border-radius: 5px;
  border-limit: corners 30%;
}

4.2. The border-clip properties

Name: border-clip, border-clip-top, border-clip-right, border-clip-bottom, border-clip-left
Value: normal | [ <length-percentage [0,∞]> | <flex> ]+
Initial: normal
Applies to: all elements
Inherited: no
Percentages: refer to length of border-edge side
Computed value: normal, or a list consisting of absolute lengths, or percentages as specified
Canonical order: per grammar
Animation type: by computed value

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

5. Drop Shadows

5.1. Coloring shadows: the box-shadow-color property

Name: box-shadow-color
Value: <color>#
Initial: currentcolor
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a computed color
Canonical order: per grammar
Animation type: by computed value

The box-shadow-color property defines one or more drop shadow colors. The property accepts a comma-separated list of shadow colors.

See the section “Layering, Layout, and Other Details” for how box-shadow-color interacts with other comma-separated drop shadow properties to form each drop shadow layer.

5.2. Offsetting shadows: the box-shadow-offset property

Name: box-shadow-offset
Value: [ none | <length>{2} ]#
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item either none or a pair of offsets (horizontal and vertical) from the element‘s box
Canonical order: per grammar
Animation type: by computed value, treating none as 0 0 when interpolated with non-none values.

The box-shadow-offset property defines one or more drop shadow offsets. The property accepts a comma-separated list. Each item in that list can either be the none value, which indicates no shadow, or a pair of horizontal and vertical offsets, where both values are described as <length> values.

none
The shadow will not be rendered. The values of other box shadow properties corresponding to this shadow have no effect.
1st <length>
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 <length>
Specifies the vertical offset of the shadow. A positive value offsets the shadow down, a negative one up.

See the section “Layering, Layout, and Other Details” for how box-shadow-offset interacts with other comma-separated drop shadow properties to form each drop shadow layer.

5.3. Blurring shadows: the box-shadow-blur property

Name: box-shadow-blur
Value: <length [0,∞]>#
Initial: 0
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a <length>
Canonical order: per grammar
Animation type: by computed value

The box-shadow-blur property defines one or more blur radii for drop shadows. The property accepts a comma-separated list of <length> values.

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.

See the section “Layering, Layout, and Other Details” for how box-shadow-blur interacts with other comma-separated drop shadow properties to form each drop shadow layer.

5.4. Spreading shadows: the box-shadow-spread property

Name: box-shadow-spread
Value: <length>#
Initial: 0
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a <length>
Canonical order: per grammar
Animation type: by computed value

The box-shadow-spread property defines one or more spread distances for drop shadows. The property accepts a comma-separated list of <length> values.

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.

See the section “Layering, Layout, and Other Details” for how box-shadow-spread interacts with other comma-separated drop shadow properties to form each drop shadow layer.

5.5. Spreading shadows: the box-shadow-position property

Name: box-shadow-position
Value: [ outset | inset ]#
Initial: outset
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item one of the keywords
Canonical order: per grammar
Animation type: by computed value

The box-shadow-position property defines one or more drop shadow positions. The property accepts a comma-separated list of outset and inset keywords.

outset
Causes the drop shadow to be an outer box-shadow. That means, one that shadows the box onto the canvas, as if it were lifted above the canvas.
inset
Causes the drop shadow to be an inner box-shadow. That means, one that shadows the canvas onto the box, as if the box were cut out of the canvas and shifted behind it.

See the section “Layering, Layout, and Other Details” for how box-shadow-position interacts with other comma-separated drop shadow properties to form each drop shadow layer.

5.6. Drop Shadows Shorthand: the box-shadow property

Name: box-shadow
Value: <spread-shadow>#
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: see individual properties
Canonical order: per grammar
Animation type: see individual properties

The box-shadow property attaches one or more drop-shadows to the box. The property accepts a comma-separated list of shadows, ordered front to back.

Each shadow is given as a <spread-shadow>, outlining the box-shadow-offset, and optional values for the box-shadow-blur, box-shadow-spread, box-shadow-color, and box-shadow-position. Omitted lengths are 0; omitted colors default to transparent when the specified offset is none and to currentcolor otherwise.

<spread-shadow> = <'box-shadow-color'>? && [ <'box-shadow-offset'> [ <'box-shadow-blur'> <'box-shadow-spread'>? ]? ] && <'box-shadow-position'>?

5.7. Layering, Layout, and Other Details

Drop shadows are declared in the coordinated value list constructed from the box-shadow-* properties, which form a coordinating list property group with box-shadow-offset as the coordinating list base property. See CSS Values 4 § A Coordinating List-Valued Properties.

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 do not trigger scrolling or increase the size of the scrollable 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.

6. Changes

6.1. Additions since [CSS3BG]

7. 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, Håkon Wium Lie, and Oriol Brufau for their suggestions and feedback specifically for this Level 4.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. URL: https://drafts.csswg.org/css-box-4/
[CSS-BREAK-4]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 4. URL: https://drafts.csswg.org/css-break-4/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. URL: https://drafts.csswg.org/css-cascade-5/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley; Lea Verou. CSS Color Module Level 4. URL: https://drafts.csswg.org/css-color/
[CSS-COLOR-5]
Chris Lilley; et al. CSS Color Module Level 5. URL: https://drafts.csswg.org/css-color-5/
[CSS-DISPLAY-4]
CSS Display Module Level 4. Editor's Draft. URL: https://drafts.csswg.org/css-display-4/
[CSS-EASING-2]
CSS Easing Functions Level 2. Editor's Draft. URL: https://drafts.csswg.org/css-easing-2/
[CSS-GRID-2]
Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Grid Layout Module Level 2. URL: https://drafts.csswg.org/css-grid-2/
[CSS-IMAGES-4]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 4. URL: https://drafts.csswg.org/css-images-4/
[CSS-RUBY-1]
Elika Etemad; et al. CSS Ruby Annotation Layout Module Level 1. URL: https://drafts.csswg.org/css-ruby-1/
[CSS-TEXT-4]
Elika Etemad; et al. CSS Text Module Level 4. URL: https://drafts.csswg.org/css-text-4/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. URL: https://drafts.csswg.org/css-values-4/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. URL: https://drafts.csswg.org/css-writing-modes-3/
[CSS-WRITING-MODES-4]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. URL: https://drafts.csswg.org/css-writing-modes-4/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. URL: https://drafts.csswg.org/css2/
[CSS3BG]
Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. URL: https://drafts.csswg.org/css-backgrounds/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119

Informative References

[CSS-LOGICAL-1]
Rossen Atanassov; Elika Etemad. CSS Logical Properties and Values Level 1. URL: https://drafts.csswg.org/css-logical-1/
[CSS1]
Håkon Wium Lie; Bert Bos. Cascading Style Sheets, level 1. 13 September 2018. REC. URL: https://www.w3.org/TR/CSS1/
[CSS3GRID]
Tab Atkins Jr.; et al. CSS Grid Layout Module Level 1. URL: https://drafts.csswg.org/css-grid/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value Logical property group
border-block <'border-block-start'> see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-block-color <'border-top-color'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-block-end <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-block-end-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-block-end-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-block-end-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-block-end-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-block-start <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-block-start-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-block-start-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-block-start-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-block-start-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-block-style <'border-top-style'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-block-width <'border-top-width'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-bottom <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-bottom-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-bottom-left-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-bottom-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-bottom-right-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-bottom-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-bottom-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-clip normal | [ <length-percentage [0,∞]> | <flex> ]+ normal all elements no refer to length of border-edge side by computed value per grammar normal, or a list consisting of absolute lengths, or percentages as specified
border-clip-bottom normal | [ <length-percentage [0,∞]> | <flex> ]+ normal all elements no refer to length of border-edge side by computed value per grammar normal, or a list consisting of absolute lengths, or percentages as specified
border-clip-left normal | [ <length-percentage [0,∞]> | <flex> ]+ normal all elements no refer to length of border-edge side by computed value per grammar normal, or a list consisting of absolute lengths, or percentages as specified
border-clip-right normal | [ <length-percentage [0,∞]> | <flex> ]+ normal all elements no refer to length of border-edge side by computed value per grammar normal, or a list consisting of absolute lengths, or percentages as specified
border-clip-top normal | [ <length-percentage [0,∞]> | <flex> ]+ normal all elements no refer to length of border-edge side by computed value per grammar normal, or a list consisting of absolute lengths, or percentages as specified
border-color [ <color> | <image-1D> ]{1,4} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-end-end-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-end-start-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-inline <'border-block-start'> see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-inline-color <'border-top-color'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-inline-end <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-inline-end-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-inline-end-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-inline-end-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-inline-end-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-inline-start <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-inline-start-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-inline-start-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-inline-start-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-inline-start-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-inline-style <'border-top-style'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-inline-width <'border-top-width'>{1,2} see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties
border-left <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-left-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-left-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-left-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-left-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-limit all | [ sides | corners ] <length-percentage [0,∞]>? | [ top | right | bottom | left ] <length-percentage [0,∞]> all all elements, except table element when border-collapse is collapse no relative to border-box discrete per grammar as specified
border-radius <length-percentage [0,∞]>{1,4} [ / <length-percentage [0,∞]>{1,4} ]? 0 all elements, except table element when border-collapse is collapse no n/a see individual properties per grammar as specified
border-right <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-right-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-right-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-right-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-right-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
border-start-end-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-start-start-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-top <line-width> || <line-style> || <color> See individual properties all elements except ruby base containers and ruby annotation containers no N/A see individual properties per grammar see individual properties
border-top-color <color> | <image-1D> currentcolor all elements except ruby base containers and ruby annotation containers no N/A see prose per grammar the computed color and/or a one-dimensional image function border-color
border-top-left-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-top-radius <length-percentage [0,∞]>{1,2} [ / <length-percentage [0,∞]>{1,2} ]? 0 all elements (but see prose) no Refer to corresponding dimension of the border box. see individual properties per grammar see individual properties
border-top-right-radius <length-percentage [0,∞]>{1,2} 0 all elements (but see prose) no Refer to corresponding dimension of the border box. by computed value per grammar pair of computed <length-percentage> values border-radius
border-top-style <line-style> none all elements except ruby base containers and ruby annotation containers no N/A discrete per grammar specified keyword border-style
border-top-width <line-width> medium all elements except ruby base containers and ruby annotation containers no N/A by computed value per grammar absolute length, snapped as a border width; zero if the border style is none or hidden border-width
box-shadow <spread-shadow># none all elements no N/A see individual properties per grammar see individual properties
box-shadow-blur <length [0,∞]># 0 all elements no N/A by computed value per grammar list, each item a <length>
box-shadow-color <color># currentcolor all elements no N/A by computed value per grammar list, each item a computed color
box-shadow-offset [ none | <length>{2} ]# none all elements no N/A by computed value, treating none as 0 0 when interpolated with non-none values. per grammar list, each item either none or a pair of offsets (horizontal and vertical) from the element‘s box
box-shadow-position [ outset | inset ]# outset all elements no N/A by computed value per grammar list, each item one of the keywords
box-shadow-spread <length># 0 all elements no N/A by computed value per grammar list, each item a <length>
corner-shape [ round | angle ]{1,4} round all elements, except table element when border-collapse is collapse no n/a discrete per grammar as specified
corners <'corner-shape'> || <'border-radius'> see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties

Issues Index

This module is currently maintained as a diff against the parts related to borders and box decorations of CSS Backgrounds and Borders Module Level 3 [CSS3BG]. 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).
How to allow custom corners? Perhaps a path() function? Or a cubic-bezier()? Something else?
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.