CSS Box Alignment Module Level 3

Abstract

This module contains the features of CSS relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout. (The alignment of text and inline-level content is defined in [CSS3TEXT] and [CSS3LINE].) CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-align” in the subject, preferably like this: “[css3-align] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The following features are at risk: …

Table of contents

• 1. Introduction
  • 1.1. Module interactions
  • 1.2. Values
• 2. Overview of Alignment Properties
• 3. Alignment Values
  • 3.1. Positional Alignment: the ‘center’, ‘stretch’, ‘start’, ‘end’, ‘self-start’, ‘self-end’, ‘flex-start’, ‘flex-end’, ‘left’, and ‘right’ keywords
  • 3.2. Baseline Alignment: the ‘baseline’ keyword
    • 3.2.1. Determining the Baseline of a Box
  • 3.3. Distributed Alignment: the ‘space-between’, ‘space-around’, and ‘space-evenly’ keywords
  • 3.4. Overflow Alignment: the ‘safe’ and ‘true’ keywords
• 4. Content Distribution: the ‘justify-content’ and ‘align-content’ properties
  • 4.1. Baseline Content-Alignment
• 5. Self-Alignment: Aligning the Box within its Parent
  • 5.1. Inline/Main-Axis Alignment: the ‘justify-self’ property
  • 5.2. Block/Cross-Axis Alignment: the ‘align-self’ property
  • 5.3. Baseline Self-Alignment
• 6. Default Alignment
  • 6.1. Inline/Main-Axis Alignment: the ‘justify-items’ property
  • 6.2. Block/Cross-Axis Alignment: the ‘align-items’ property
• 7. Changes
• 8. Conformance
  • 8.1. Document conventions
  • 8.2. Conformance classes
  • 8.3. Partial implementations
  • 8.4. Experimental implementations
  • 8.5. Non-experimental implementations
  • 8.6. CR exit criteria
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index

1. Introduction

This section is not normative.

CSS Levels 1 and 2 allowed for the alignment of text via ‘text-align’ and the alignment of blocks by balancing ‘auto’ margins. However, except in table cells, vertical alignment was not possible. As CSS3 adds further capabilities, the ability to align boxes in various dimensions becomes more critical. This module attempts to create a cohesive and common box alignment model to share among all of CSS.

The alignment of text and inline-level content is defined in [CSS3TEXT] and [CSS3LINE].

Inspiration for this document:

• summary of a discussion for implementing <CENTER>
• Minutes from March 2008 F2F
• fantasai's attempt to merge all alignment properties

1.1. Module interactions

This module adds some new alignment capabilities to the block layout model described in [CSS21] chapters 9 and 10 and defines the interaction of these properties with the alignment of table cell content using ‘vertical-align’, as defined in [CSS21] chapter 17. The interaction of these properties with Grid Layout [CSS3-GRID-LAYOUT] and Flexible Box Layout [CSS3-FLEXBOX] is defined in their respective modules.

No properties in this module apply to the ::first-line or ::first-letter pseudo-elements.

1.2. Values

This specification follows the CSS property definition conventions from [CSS21]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3VAL], when combined with this module, adds the ‘initial’ keyword as a possible property value.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

2. Overview of Alignment Properties

The alignment properties in CSS can be described along two axes:

• which dimension they apply to (inline vs. stacking), and
• whether they control the position of the box within its parent, or the box's content within itself.

This proposal uses the terms ‘justify’ and ‘align’ to distinguish between alignment in the inline and stacking dimensions, respectively. The choice is somewhat arbitrary, but having the two terms allows for a consistent naming scheme that works across all of CSS's layout models.

The following table summarizes the proposed alignment properties and the display types they can apply to.

Common	Axis	Aligns	Applies to
‘justify-content’	inline	content within element (effectively adjusts padding)	block containers and flex containers
‘align-content’	stacking		block containers and flex containers
‘justify-self’	inline	element within parent (effectively adjusts margins)	block-level elements and grid items
‘align-self’	stacking		flex items and grid items
‘justify-items’	inline	items inside element (controls child items’ ‘align/justify-self: auto’)	grid containers
‘align-items’	stacking		flex containers and grid containers

The exact definition of these properties outside what's what's defined in Flexbox is still being worked out. This is a second Working Draft and is NOT STABLE.

The ‘-items’ values don't affect the element itself. When set on a flex container or grid container, they specify the interpretation of any ‘align/justify-self: auto’ used on the items in the container element.

3. Alignment Values

All of the alignment properties use a common set of values, defined below.

alignment subject

The alignment subject is the thing or things being aligned by the property. For ‘justify-self’ and ‘align-self’, the alignment subject is the margin box of the box the property is set on. For ‘justify-content’ and ‘align-content’, the alignment subject is defined by the layout mode.

alignment container

The alignment container is the rectangle that the alignment subject is aligned within. This is defined by the layout mode, but is usually the alignment subject’s containing block.

3.1. Positional Alignment: the ‘center’, ‘stretch’, ‘start’, ‘end’, ‘self-start’, ‘self-end’, ‘flex-start’, ‘flex-end’, ‘left’, and ‘right’ keywords

The positional alignment keywords specify a position for an alignment subject with respect to its alignment container.

The <item-position> set of values is used by ‘justify-self’ and ‘align-self’ to align the box within its alignment container, and also by ‘justify/align-items’ (to specify default values for ‘justify/align-self’). The <content-position> set of values is used by ‘justify-content’ and ‘align-content’ to align the box's contents within itself.

<item-position> = center | start | end | self-start | self-end | 
                  flex-start | flex-end | left | right;
<content-position> = center | start | end | flex-start | flex-end | left | right;

Values have the following meanings:

‘center’

Centers the alignment subject within its alignment container.

‘stretch’

If the ‘width’ or ‘height’ (as appropriate) of the alignment subject is ‘auto’, its used value is the length necessary to make the alignment subject’s outer size as close to the size of the alignment container as possible, while still respecting the constraints imposed by ‘min/max-width/height’. Otherwise, this is equivalent to ‘start’.

‘start’

Aligns the alignment subject to be flush with the alignment container’s start edge.

‘end’

Aligns the alignment subject to be flush with the alignment container’s end edge.

‘self-start’

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s start side.

‘self-end’

Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the alignment subject’s end side.

‘flex-start’

Only used in flex layout. [CSS3-FLEXBOX] Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the flex container’s main-start or cross-start side, as appropriate. When used in layout modes other than Flexbox, this is equivalent to ‘start’.

‘flex-end’

Only used in flex layout. Aligns the alignment subject to be flush with the edge of the alignment container corresponding to the flex container’s main-end or cross-end side, as appropriate. When used in layout modes other than Flexbox, this is equivalent to ‘start’.

‘left’

Aligns the alignment subject to be flush with the alignment container’s line-left edge. If the property's axis is not parallel with the inline axis, this is equivalent to ‘start’.

‘right’

Aligns the alignment subject to be flush with the alignment container’s line-right edge. If the property's axis is not parallel with the inline axis, this is equivalent to ‘start’.

Add example images.

3.2. Baseline Alignment: the ‘baseline’ keyword

Baseline alignment is a form of positional alignment that relies on aligning the baselines of related alignment subjects to each other.

‘baseline’

For table cells, grid items, and flex items, aligns the cell/item's first formatted line's baseline, if any, with the same baseline in other ‘baseline’-aligned cells/items in the row/column (as appropriate for the axis).

In the case of ‘align-content’/‘justify-content’, this shifts the content of the box within it, and may also affect its sizing. In the case of ‘align-self’/‘justify-self’ this shifts the entire box within its container, and may affect the sizing of its container.

If the alignment subject’s position is not fully determined by baseline alignment, the content is start-aligned insofar as possible while preserving the baseline alignment. (Content that has no first-line baseline is thus start-aligned.)

Add example images.

3.2.1. Determining the Baseline of a Box

The baseline of a box is determined differently based on the layout model, as follow:

‘block’

‘list-item’

The inline-axis baseline of a block is the baseline of the first in-flow line box in the block, or the first in-flow block-level child in the block that has a baseline, whichever comes first. If there is no such line box or child, then the block has no baseline. For the purposes of finding a baseline, in-flow boxes with a scrolling mechanisms (see the ‘overflow’ property) must be considered as if scrolled to their origin position.

A block has no block-axis baseline.

‘table’

The inline-axis baseline of a table box is the baseline of its first row. However, when calculating the baseline of an inline-block, table boxes must be skipped.

The block-axis baseline of a table is undefined.

Or does it have no baseline? Or is it based on its first column?

‘table-row’

If any cells in the row participate in its baseline alignment, the inline-axis baseline of the row is their baseline, after baseline alignment has been performed. Otherwise, the inline-axis baseline of the row is the bottom content edge of the lowest cell in the row. [CSS21]

A table row has no block-axis baseline.

‘table-cell’

The inline-axis baseline of a table cell is the baseline of the first in-flow line box in the cell, or the first in-flow element that contributes a baseline in that axis, whichever comes first.

A table cell has no block-axis baseline.

‘flex’

See Flex Baselines in [CSS3-FLEXBOX].

‘grid’

See Grid Baselines in [CSS3-GRID-LAYOUT].

Maybe these things are wrong? CSS 2.1 is really weird about baseline alignment. This whole section needs to be better defined and reviewed.

3.3. Distributed Alignment: the ‘space-between’, ‘space-around’, and ‘space-evenly’ keywords

The distribution values are used by ‘justify-content’ and ‘align-content’ to distribute the items in the alignment subject evenly between the start and end edges of the alignment container. When the alignment subject cannot be distributed in this way, they behave as their fallback alignment. Each distribution value has an associated <content-position> as a fallback alignment, but one can alternatively be explicitly specified in the property.

<content-distribution> = space-between | space-around | space-evenly | stretch

‘space-between’

The items are evenly distributed in the alignment container. The first item is placed flush with the start edge of the alignment container, the last item is placed flush with the end edge of the alignment container, and the remaining items are distributed so that the spacing between any two adjacent items is the same. Unless otherwise specified, this value falls back to ‘start’.

‘space-around’

The items are evenly distributed in the alignment container, with a half-size space on either end. The items are distributed so that the spacing between any two adjacent items is the same, and the spacing before the first and after the last item is half the size of the other spacing. Unless otherwise specified, this value falls back to ‘center’.

‘space-evenly’

The items are evenly distributed in the alignment container, with a full-size space on either end. The items are distributed so that the spacing between any two adjacent items, before the first item, and after the last item is the same. Unless otherwise specified, this value falls back to ‘center’.

‘stretch’

If the combined size of the items is less than the size of the alignment container, any ‘auto’-sized items have their size increased equally so that the combined size exactly fills the alignment container. Otherwise, or if there are no ‘auto’-sized items, this value is identical to ‘flex-start’. (For layout modes other than flex layout, ‘flex-start’ is identical to ‘start’.)

Add example images.

3.4. Overflow Alignment: the ‘safe’ and ‘true’ keywords

When the alignment subject is larger than the alignment container, it will overflow. Some alignment modes, if honored in this situation, may cause data loss: for example, if the contents of a sidebar are centered, when they overflow they may send part of their boxes past the viewport's start edge, which can't be scrolled to.

To help combat this problem, an overflow alignment mode can be explicitly specified. "True" alignment honors the specified alignment mode in overflow situations, even if it causes data loss, while "safe" alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.

If the overflow alignment isn't explicitly specified, the default overflow alignment is determined by the layout mode. Document-centric layout modes, such as block layout, default to "safe" overflow alignment, while design-centric layout modes, such as flex layout, default to "true" overflow alignment.

<overflow-position> = true | safe

‘safe’

If the size of the alignment subject overflows the alignment container, the alignment subject is instead aligned as if the alignment mode were ‘start’.

‘true’

Regardless of the relative sizes of the alignment subject and alignment container, the given alignment value is honored.

Transplant example 10 from flexbox.

4. Content Distribution: the ‘justify-content’ and ‘align-content’ properties

The ‘justify-content’ and ‘align-content’ properties control alignment of the box's content within the box.

Aligns the contents of the box as a whole along the box's inline/row/main axis. Values other than ‘auto’ are defined above. If both a <content-distribution> and <content-position> are given, the second value represents an explicit fallback alignment.

Block Containers:

The alignment container is the block container’s content box. The alignment subject is the entire contents of the block.

The ‘align-content’ property applies along the block axis, but if a <content-distribution> is specified the fallback alignment is used instead. The ‘justify-content’ property does not apply to and has no effect on block containers.

All values other than ‘auto’ force the block container to establish a new formatting context. For table cells, the behavior of the ‘auto’ depends on the computed value of ‘vertical-align’: ‘top’ makes it behave as ‘start’, ‘middle’ makes it behave as ‘center’, ‘bottom’ makes it behave as ‘end’, and all other values make it behave as ‘baseline’. ‘auto’ otherwise behaves as ‘start’.

Multicol Layout:

The alignment container is the multi-column element’s content box. The alignment subject is the column boxes, as a unit.

The ‘align-content’ property applies along the block axis, but if a <content-distribution> is specified the fallback alignment is used instead. The ‘justify-content’ property does not apply to and has no effect on multi-column elements.

‘auto’ behaves as ‘start’.

Flex Containers:

‘auto’ computes to ‘stretch’.

The alignment container is the flex container’s content box. For ‘justify-content’, the alignment subject is the flex items in each flex line; for ‘align-content’, the alignment subject is the flex lines.

The ‘align-content’ property applies along the cross axis. The ‘justify-content’ property applies along the main axis, but ‘stretch’ behaves as ‘start’.

See [CSS3-FLEXBOX] for details.

Grid Containers:

‘auto’ computes to ‘start’, and ‘stretch’ behaves like ‘start’.

The alignment container is the grid container’s content box. The alignment subject is the bounds of the grid. Need to dfn a better term for this in Grid.

The ‘align-content’ property applies along the block (column) axis. The ‘justify-content’ property applies along the inline (row) axis. In both properties, if a <content-distribution> is specified, the fallback alignment is used instead.

4.1. Baseline Content-Alignment

The content of boxes participating in row-like layout contexts can be baseline-aligned to each other. The alignment subject of a box participating in baseline content-alignment is shifted such that the baseline of the first formatted line of all participating boxes align, and the intrinsic size of each box is increased by the distance the content needed to shift in order to thus align the baselines.

The set of boxes that participate in baseline content-alignment together depends on the layout model:

Table Cells:

A table cell participates in baseline content-alignment in either its row or column (whichever matches its inline axis) if its computed ‘align-content’ is ‘baseline’.

Flex Items:

A flex item participates in baseline content-alignment in its flex line if its computed ‘align-content’ is ‘baseline’, its computed ‘align-self’ is ‘stretch’ or ‘start’, and its inline axis is parallel to the main axis.

Grid Items:

A grid item participates in baseline content-alignment in either its row or column (whichever matches its inline axis) if its computed ‘align-content’ is ‘baseline’, and its computed ‘align-self’ or ‘justify-self’ (whichever affects its block axis) is ‘stretch’ or ‘start’.

If a cell/item spans multiple rows or columns, it participates in alignment within its start-most row/column only.

5. Self-Alignment: Aligning the Box within its Parent

The ‘justify-self’ and ‘align-self’ properties control alignment of the box within its containing block.

5.1. Inline/Main-Axis Alignment: the ‘justify-self’ property

Justifies the box within its parent along the inline/row/main axis: the box's outer edges are aligned within its alignment container as described by its alignment value.

The ‘auto’ keyword computes to ‘stretch’ on absolutely-positioned elements, and to the computed value of ‘justify-items’ on the parent (minus any ‘legacy’ keywords) on all other boxes.

Block-level Boxes:

The ‘justify-self’ property applies along its containing block's inline axis.

The alignment container is the block's containing block except that for block-level elements that establish a block formatting context and are placed next to a float, the alignment container is reduced by the space taken up by the float. (Note: This is the legacy behavior of HTML align.) The alignment subject is the block's margin box.

The default overflow alignment is ‘safe’. In terms of CSS2.1 block-level formatting [CSS21], the rules for "over-constrained" computations in section 10.3.3 are ignored in favor of alignment as specified here and the used value of the offset properties are not adjusted to correct for the over-constraint.

The ‘stretch’ value is equivalent to ‘start’ on block-level boxes.

This property does not apply to floats.

Absolutely-positioned Boxes:

The ‘justify-self’ property applies along its containing block's inline axis.

When neither margin in this dimension is ‘auto’ and neither offset property in this dimension is ‘auto’, values other than ‘stretch’ cause non-replaced absolutely-positioned boxes to use shrink-to-fit sizing for calculating ‘auto’ measures, and ‘justify-self’ dictates alignment as follows:

The alignment container is the box's containing block as modified by the offset properties (‘top’/‘right’/‘bottom’/‘left’). The alignment subject is the box's margin box.

The default overflow alignment is ‘safe’. In terms of CSS2.1 formatting [CSS21], the rules for "over-constrained" computations in section 10.3.7 are ignored in favor of alignment as specified here and the used value of the offset properties are not adjusted to correct for the over-constraint.

The ‘stretch’ keyword is equivalent to ‘start’ on replaced absolutely-positioned boxes. (This is because CSS 2.1 does not stretch replaced elements to fit into fixed offsets.)

Table Cells:

This property does not apply to table cells, because their position and size is fully constrained by table layout.

Flex Items:

This property does not apply to flex items, because there is more than one item in the main axis. See ‘flex’ for stretching and ‘justify-content’ for main-axis alignment. [CSS3-FLEXBOX]

Grid Items:

The ‘justify-self’ property applies along the grid's row axis.

The alignment container is the grid cell. The alignment subject is the grid item’s margin box. The default overflow alignment is ‘true’.

Should we make ‘stretch’ actually work on block-level/abspos replaced boxes? To maintain legacy compat, this requires adding a new value (named ‘normal’?) which has the current behavior of stretching non-replaced boxes and start-aligning replaced ones.

5.2. Block/Cross-Axis Alignment: the ‘align-self’ property

Aligns the box within its parent along the block/column/cross axis: the box's outer edges are aligned within its alignment container as described by its alignment value.

The ‘auto’ keyword computes to ‘stretch’ on absolutely-positioned elements, and to the computed value of ‘justify-items’ on the parent (minus any ‘legacy’ keywords) on all other boxes.

Block-level Boxes:

The ‘align-self’ property does not apply to block-level boxes (including floats), because there is more than one item in the block axis.

Absolutely-positioned Boxes:

The ‘justify-self’ property applies along its containing block's inline axis.

When neither margin in this dimension is ‘auto’ and neither offset property in this dimension is ‘auto’, values other than ‘stretch’ cause non-replaced absolutely-positioned boxes to use shrink-to-fit sizing for calculating ‘auto’ measures, and ‘align-self’ dictates alignment as follows:

The alignment container is the box's containing block as modified by the offset properties (‘top’/‘right’/‘bottom’/‘left’). The alignment subject is the box's margin box.

The default overflow alignment is ‘safe’. In terms of CSS2.1 formatting [CSS21], the rules for "over-constrained" computations in section 10.6.4 are ignored in favor of alignment as specified here and the used value of the offset properties are not adjusted to correct for the over-constraint.

The ‘stretch’ keyword is equivalent to ‘start’ on replaced absolutely-positioned boxes. (This is because CSS 2.1 does not stretch replaced elements to fit into fixed offsets.)

Table Cells:

This property does not apply to table cells, because their position and size is fully constrained by table layout.

Flex Items:

The ‘align-self’ property applies along the flexbox's cross axis.

The alignment container is the flex line the item is in. The alignment subject is the flex item’s margin box. The default overflow alignment is ‘true’. See [CSS3-FLEXBOX] for details.

Grid Items:

The ‘align-self’ property applies along the grid's column axis.

The alignment container is the grid cell. The alignment subject is the grid item’s margin box. The default overflow alignment is ‘true’.

5.3. Baseline Self-Alignment

Boxes participating in row-like layout contexts can be baseline-aligned to each other. An alignment subject participating in baseline content-alignment is shifted such that the baseline of the first formatted line of all participating boxes align, and the intrinsic size of its alignment container is increased by the largest distance a box needed to shift in order to thus align the baselines.

The set of boxes that participate in baseline self-alignment together depends on the layout model:

Flex Items:

A flex item participates in baseline content-alignment if its inline axis is parallel to the main axis. See [CSS3-FLEXBOX] for details.

Grid Items:

A grid item participates in baseline content-alignment in either its row or column (whichever matches its inline axis) if its ‘justify-self’ or ‘align-self’ property (whichever matches its inline axis) computes to ‘baseline’.

If a cell/item spans multiple rows or columns, it participates in alignment within its start-most row/column only.

6. Default Alignment

The ‘align-items’ and ‘justify-items’ properties set the default ‘align-self’ and ‘justify-self’ behavior of the items contained by the element.

6.1. Inline/Main-Axis Alignment: the ‘justify-items’ property

This property specifies the default ‘justify-self’ for all of the boxes (including anonymous boxes) participating in this box's formatting context. Values have the following meanings:

‘auto’

If the element has a parent, and its computed value for ‘justify-items’ includes the ‘legacy’ keyword, ‘auto’ computes to the parent's value.

Otherwise, ‘auto’ computes to:

• ‘stretch’ for flex containers and grid containers
• ‘start’ for everything else

‘legacy’

This keyword causes the value to effectively inherit into descendants. It can only be combined with the ‘center’, ‘left’, and ‘right’ positions. The alignment keyword, but not the ‘legacy’ keyword, is passed to ‘justify-self’. It exists to implement the legacy alignment behavior of HTML's <center> element and align attribute.

Other values have no special handling and are merely passed to ‘justify-self’.

6.2. Block/Cross-Axis Alignment: the ‘align-items’ property

This property specifies the default ‘align-self’ for all of the boxes (including anonymous boxes) participating in this box's formatting context. Values have the following meanings:

‘auto’

Computes to:

• ‘stretch’ for flex containers and grid containers
• ‘start’ for everything else

Other values have no special handling and are merely passed to ‘align-self’.

7. Changes

Changes since the First Public Working Draft include:

• Defined terms alignment container and alignment subject to make ti easier to define alignment.
• Pulled out all value definitions into a general section on Alignment Values.
• Added both ‘safe’ and ‘true’ as overflow alignment keywords and made the default layout-mode-dependent.
• Removed ‘head’ and ‘foot’ keywords, defined ‘start’ and ‘end’ keywords as relative to the alignment container‘s writing mode, and added ’‘self-start’‘ and ’‘self-end’‘ to be relative to the alignment subject.
• Filled out details on baseline alignment.
• Made ’justify-items' handle the <CENTER>>/align attribute requirements by adding ‘legacy’ keyword.
• Added fallback alignment ability for ‘justify-content’ and ‘align-content’.
• Defined alignment behavior of absolutely-positioned elements.
• Improved details on alignment behavior of other layout modes.

8. Conformance

8.1. 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:

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.

8.2. Conformance classes

Conformance to CSS Box Alignment Module Level 3 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 CSS Box Alignment Module Level 3 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 CSS Box Alignment Module Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Box Alignment Module Level 3 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 CSS Box Alignment Module Level 3 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.

8.3. 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.

8.4. Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

8.5. 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 https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

8.6. CR exit criteria

[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Acknowledgments

Special thanks goes to Markus Mielke, Alex Mogilevsky, and the participants in the CSSWG's March 2008 F2F alignment discussions.

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: https://www.w3.org/TR/2011/REC-CSS2-20110607

[CSS3-FLEXBOX]

Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. CSS Flexible Box Layout Module. 18 September 2012. W3C Candidate Recommendation. (Work in progress.) URL: https://www.w3.org/TR/2012/CR-css3-flexbox-20120918/

[CSS3-GRID-LAYOUT]

Alex Mogilevsky; et al. CSS Grid Layout. 6 November 2012. W3C Working Draft. (Work in progress.) URL: https://www.w3.org/TR/2012/WD-css3-grid-layout-20121106/

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: https://www.ietf.org/rfc/rfc2119.txt

Other references

[CSS3LINE]

Michel Suignard; Eric A. Meyer. CSS3 module: line. 15 May 2002. W3C Working Draft. (Work in progress.) URL: https://www.w3.org/TR/2002/WD-css3-linebox-20020515

[CSS3TEXT]

Elika J. Etemad; Koji Ishii. CSS Text Module Level 3. 13 November 2012. W3C Working Draft. (Work in progress.) URL: https://www.w3.org/TR/2012/WD-css3-text-20121113/

[CSS3VAL]

Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 28 August 2012. W3C Candidate Recommendation. (Work in progress.) URL: https://www.w3.org/TR/2012/CR-css3-values-20120828/

Index

• align-content, 4.
• align-items, 6.2.
• alignment container, 3.
• alignment subject, 3.
• align-self, 5.2.
• authoring tool, 8.2.
• ‘baseline’, 3.2.
• baseline content-alignment, 4.1.
• baseline self-alignment, 5.3.
• ‘center’, 3.1.
• <content-distribution>, 3.3.
• <content-position>, 3.1.
• ‘end’, 3.1.
• fallback alignment, 3.3.
• ‘flex-end’, 3.1.
• ‘flex-start’, 3.1.
• <item-position>, 3.1.
• justify-content, 4.
• justify-items, 6.1.
• justify-self, 5.1.
• ‘left’, 3.1.
• ‘legacy’, 6.1.
• overflow alignment, 3.4.
• <overflow-position>, 3.4.
• renderer, 8.2.
• ‘right’, 3.1.
• ‘safe’, 3.4.
• ‘self-end’, 3.1.
• ‘self-start’, 3.1.
• ‘space-around’, 3.3.
• ‘space-between’, 3.3.
• ‘space-evenly’, 3.3.
• ‘start’, 3.1.
• ‘stretch’, 3.1., 3.3.
• style sheet
  • as conformance class, 8.2.
• ‘true’, 3.4.

Property index