CSS Display Module Level 3

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-display/
Latest published version:
https://www.w3.org/TR/css-display-3/
Previous Versions:
Implementation Report:
https://wpt.fyi/results/css/css-display?label=master&label=experimental&aligned
Test Suite:
http://test.csswg.org/suites/css-display-3_dev/nightly-unstable/
Issue Tracking:
CSSWG Issues Repository
Editors:
Tab Atkins Jr. (Google)
Elika J. Etemad / fantasai (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This module describes how the CSS formatting box tree is generated from the document element tree and defines the display property that controls it.

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-display” in the title, like this: “[css-display] …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 15 September 2020 W3C Process Document.

A preliminary implementation report is available. Further tests will be added during the CR period.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

This section is normative.

CSS takes a source document organized as a tree of elements (which can contain a mix of other elements and text nodes) and text nodes (which can contain text), and renders it onto a canvas such as your screen, a piece of paper, or an audio stream. Although any such source document can be rendered with CSS, the most commonly used type is the the DOM. [DOM] (Some of these more complex tree types might have additional types of nodes, such as the comment nodes in the DOM. For the purposes of CSS, all of these additional types of nodes are ignored, as if they didn’t exist.)

To do this, it generates an intermediary structure, the box tree, which represents the formatting structure of the rendered document. Each box in the box tree represents its corresponding element (or pseudo-element) in space and/or time on the canvas, while each text run in the box tree likewise represents the contents of its corresponding text nodes.

To create the box tree, CSS first uses cascading and inheritance, to assign a computed value for each CSS property to each element and text node in the source tree. (See [CSS-CASCADE-3].)

Then, for each element, CSS generates zero or more boxes as specified by that element’s display property. Typically, an element generates a single box, the principal box, which represents itself and contains its contents in the box tree. However, some display values (e.g. display: list-item) generate more than one box (e.g. a principal block box and a child marker box). And some values (such as none or contents) cause the element and/or its descendants to not generate any boxes at all. Boxes are often referred to by their display type—e.g. a box generated by an element with display: block is called a “block box” or just a “block”.

A box is assigned the same styles as its generating element, unless otherwise indicated. In general, inherited properties are assigned to the principal box, and then inherit through the box tree to any other boxes generated by the same element. Non-inherited properties default to applying to the principal box, but when the element generates multiple boxes, are sometimes defined to apply to a different box: for example, the border properties applied to a table element are applied to its table grid box, not to its principal table wrapper box. If the value computation process alters the styles of those boxes, and the element’s style is requested (such as through getComputedStyle()), the element reflects, for each property, the value from the box to which that property was applied.

Similarly, each contiguous sequence of sibling text nodes generates a text run containing their text contents, which is assigned the same styles as the generating text nodes. If the sequence contains no text, however, it does not generate a text run.

In constructing the box tree, boxes generated by an element are descendants of the principal box of any ancestor elements. In the general case, the direct parent box of an element’s principal box is the principal box of its nearest ancestor element that generates a box; however, there are some exceptions, such as for run-in boxes, display types (like tables) that generate multiple container boxes, and intervening anonymous boxes.

An anonymous box is a box that is not associated with any element. Anonymous boxes are generated in certain circumstances to fix up the box tree when it requires a particular nested structure that is not provided by the boxes generated from the element tree. For example, a table cell box requires a particular type of parent box (the table row box), and will generate an anonymous table row box around itself if its parent is not a table row box. (See [CSS2] § 17.2.1.) Unlike element-generated boxes, whose styles inherit strictly through the element tree, anonymous boxes (which only exist in the box tree) inherit through their box tree parentage.

In the course of layout, boxes and text runs can be broken into multiple fragments. This happens, for example, when an inline box and/or text run is broken across lines, or when a block box is broken across pages or columns, in a process called fragmentation. It can also happen due to bidi reordering of text (see Applying the Bidirectional Reordering Algorithm in CSS Writing Modes) or higher-level display type box splitting, e.g. block-in-inline splitting (see CSS2§9.2) or column-spanner-in-block splitting (see CSS Multi-column Layout). A box therefore consists of one or more box fragments, and a text run consists of one or more text fragments. See [CSS-BREAK-3] for more information on fragmentation.

Note: Many of the CSS specs were written before this terminology was ironed out, or refer to things incorrectly, so view older specs with caution when they’re using these terms. It should be possible to infer from context which term they really mean. Please report errors in specs when you find them, so they can be corrected.

Note: Further information on the “aural” box tree and its interaction with the display property can be found in the CSS Speech Module. [CSS-SPEECH-1]

1.1. Module interactions

This module replaces and extends the definition of the display property defined in [CSS2] section 9.2.4.

None of the properties in this module apply to the ::first-line or ::first-letter pseudo-elements.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Box Layout Modes: the display property

display

In all current engines.

Firefox1+Safari1+Chrome1+
Opera7+Edge79+
Edge (Legacy)12+IE4+
Firefox for Android4+iOS Safari1+Chrome for Android18+Android WebView37+Samsung Internet1.0+Opera Mobile10.1+
Name: display
Value: [ <display-outside> || <display-inside> ] | <display-listitem> | <display-internal> | <display-box> | <display-legacy>
Initial: inline
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: a pair of keywords representing the inner and outer display types plus optional list-item flag, or a <display-internal> or <display-box> keyword; see prose in a variety of specs for computation rules
Canonical order: per grammar
Animation type: not animatable

User agents are expected to support this property on all media, including non-visual ones. The display property defines an element’s display type, which consists of the two basic qualities of how an element generates boxes:

Text runs have no display type.

Some display values have additional side-effects: such as list-item, which also generates a ::marker pseudo-element, and none, which causes the element’s entire subtree to be left out of the box tree.

The display property has no effect on an element’s semantics: these are defined by the document language and are not affected by CSS. Aside from the none value, which also affects the aural/speech output [CSS-SPEECH-1] and interactivity of an element and its descendants, the display property only affects visual layout: its purpose is to allow designers freedom to change the layout behavior of an element without affecting the underlying document semantics.

Values are defined as follows:

display-outside

In all current engines.

Firefox1+Safari1+Chrome1+
Opera7+Edge79+
Edge (Legacy)12+IE4+
Firefox for Android4+iOS Safari1+Chrome for Android18+Android WebView1+Samsung Internet1.0+Opera Mobile10.1+
<display-outside>  = block | inline | run-in
<display-inside>   = flow | flow-root | table | flex | grid | ruby
<display-listitem> = <display-outside>? && [ flow | flow-root ]? && list-item
<display-internal> = table-row-group | table-header-group |
                     table-footer-group | table-row | table-cell |
                     table-column-group | table-column | table-caption |
                     ruby-base | ruby-text | ruby-base-container |
                     ruby-text-container
<display-box>      = contents | none
<display-legacy>   = inline-block | inline-table | inline-flex | inline-grid

The following informative table summarizes the values of display:

Short display Full display Generated box
none subtree omitted from box tree
contents element replaced by contents in box tree
block block flow block-level block container aka block box
flow-root block flow-root block-level block container that establishes a new block formatting context (BFC)
inline inline flow inline box
inline-block inline flow-root inline-level block container aka inline block
run-in run-in flow run-in box (inline box with special box-tree-munging rules)
list-item block flow list-item block box with additional marker box
inline list-item inline flow list-item inline box with additional marker box
flex block flex block-level flex container
inline-flex inline flex inline-level flex container
grid block grid block-level grid container
inline-grid inline grid inline-level grid container
ruby inline ruby inline-level ruby container
block ruby block ruby block box containing ruby container
table block table block-level table wrapper box containing table grid box
inline-table inline table inline-level table wrapper box containing table grid box
<display-internal> types layout-specific internal box

Note: Following the precedence rules of “most backwards-compatible, then shortest”, serialization of equivalent display values uses the “Short display” column. [CSSOM]

2.1. Outer Display Roles for Flow Layout: the block, inline, and run-in keywords

The <display-outside> keywords specify the element’s outer display type, which is essentially its principal box’s role in flow layout. They are defined as follows:

block
The element generates a box that is block-level when placed in flow layout. [CSS2]
inline
The element generates a box that is inline-level when placed in flow layout. [CSS2]
run-in
The element generates an run-in box, which is a type of inline-level box with special behavior that attempts to merge it into a subsequent block container. See § 5 Run-In Layout for details.

Note: Outer display types do affect replaced elements.

If a <display-outside> value is specified but <display-inside> is omitted, the element’s inner display type defaults to flow.

2.2. Inner Display Layout Models: the flow, flow-root, table, flex, grid, and ruby keywords

The <display-inside> keywords specify the element’s inner display type, which defines the type of formatting context that lays out its contents (assuming it is a non-replaced element). They are defined as follows:

flow
The element lays out its contents using flow layout (block-and-inline layout).

If its outer display type is inline or run-in, and it is participating in a block or inline formatting context, then it generates an inline box.

Otherwise it generates a block container box.

Depending on the value of other properties (such as position, float, or overflow) and whether it is itself participating in a block or inline formatting context, it either establishes a new block formatting context for its contents or integrates its contents into its parent formatting context. See CSS2.1 Chapter 9. [CSS2] A block container that establishes a new block formatting context is considered to have a used inner display type of flow-root.

flow-root
The element generates a block container box, and lays out its contents using flow layout. It always establishes a new block formatting context for its contents. [CSS2]
table
The element generates a principal table wrapper box that establishes a block formatting context, and which contains an additionally-generated table grid box that establishes a table formatting context. [CSS2]
flex
The element generates a principal flex container box and establishes a flex formatting context. [CSS-FLEXBOX-1]
grid
The element generates a principal grid container box, and establishes a grid formatting context. [CSS-GRID-1]

(Grids using subgrid might not generate a new grid formatting context; see [CSS-GRID-2] for details.)

ruby
The element generates a ruby container box and establishes a ruby formatting context in addition to integrating its base-level contents into its parent formatting context (if it is inline) or generating a wrapper box of the appropriate outer display type (if it is not). [CSS-RUBY-1]

If a <display-inside> value is specified but <display-outside> is omitted, the element’s outer display type defaults to blockexcept for ruby, which defaults to inline.

2.3. Generating Marker Boxes: the list-item keyword

The list-item keyword causes the element to generate a ::marker pseudo-element [CSS-PSEUDO-4] with the content specified by its list-style properties (CSS 2.1§12.5 Lists) [CSS2] together with a principal box of the specified type for its own contents.

If no inner display type value is specified, the principal box’s inner display type defaults to flow. If no outer display type value is specified, the principal box’s outer display type defaults to block.

Note: In this level, as restricted in the grammar, list-items are limited to the Flow Layout display types (block/inline/run-in with flow/flow-root inner types). This restriction may be relaxed in a future level of this module.

2.4. Layout-Internal Display Types: the table-* and ruby-* keywords

Some layout models, such as table and ruby, have a complex internal structure, with several different roles that their children and descendants can fill. This section defines those “layout-internaldisplay values, which only have meaning within that particular layout mode.

Unless otherwise specified, both the inner display type and the outer display type of elements using these display values are set to the given keyword.

When the display property of a replaced element computes to one of the layout-internal values, it is handled as having a used value of inline. White space collapsing and anonymous box generation must happen around those replaced elements based on that inline value, as if they never had a layout-internal display value applied to them.

Authors should not assign a layout-internal display value to replaced elements.

The <display-internal> keywords are defined as follows:

table-row-group, table-header-group, table-footer-group, table-row, table-cell, table-column-group, table-column
The element is an internal table element. It generates the appropriate internal table box which participates in a table formatting context. See CSS2§17.2 [CSS2].

table-cell boxes have a flow-root inner display type.

table-caption
The element generates a table caption box, which is a block box with special behavior with respect to table and table wrapper boxes. See CSS2§17.2 [CSS2].

table-caption boxes have a flow-root inner display type.

ruby-base, ruby-text, ruby-base-container, ruby-text-container
The element is an internal ruby element. It generates the appropriate internal ruby box which participates in a ruby formatting context. [CSS-RUBY-1]

ruby-base and ruby-text have a flow inner display type.

Boxes with layout-specific display types generate anonymous wrapper boxes around themselves when placed in an incompatible parent, as defined by their respective specifications.

For example, Table Layout requires that a table-cell box must have a table-row parent box.

If it is misparented, like so:

<div style="display:block;">
  <div style="display:table-cell">...</div>
</div>

It will generate wrapper boxes around itself, producing a structure like:

block box
└anonymous table box
 └anonymous table-row-group box
  └anonymous table-row box
   └table-cell box

Even if the parent is another internal table element, if it’s not the correct one, wrapper boxes will be generated. For example, in the following markup:

<div style="display:table;">
  <div style="display:table-row">
    <div style="display:table-cell">...</div>
  </div>
</div>

Anonymous wrapper box generation will produce:

table box
└anonymous table-row-group box
 └table-row box
  └table-cell box

This "fix-up" ensures that table layout has a predictable structure to operate on.

2.5. Box Generation: the none and contents keywords

While display can control the types of boxes an element will generate, it can also control whether an element will generate any boxes at all.

The <display-box> keywords are defined as follows:

contents
The element itself does not generate any boxes, but its children and pseudo-elements still generate boxes and text runs as normal. For the purposes of box generation and layout, the element must be treated as if it had been replaced in the element tree by its contents (including both its source-document children and its pseudo-elements, such as ::before and ::after pseudo-elements, which are generated before/after the element’s children as normal).

Note: As only the box tree is affected, any semantics based on the document tree, such as selector-matching, event handling, and property inheritance, are not affected. As of writing, however, this is not implemented correctly in major browsers, so using this feature on the Web must be done with care as it can prevent accessibility tools from accessing the element’s semantics.

This value computes to display: none on replaced elements and other elements whose rendering is not entirely controlled by CSS; see Appendix B: Effects of display: contents on Unusual Elements for details.

Note: Replaced elements and form controls are treated specially because removing only the element’s own generating box is a more-or-less undefined operation. As this behavior may be refined if use cases (and more precise rendering models) develop, authors should use display: none rather than display: contents on such elements for forward-compatibility.

none
The element and its descendants generate no boxes or text runs.

Similarly, if a text node is defined to behave as display: none, it generates no text runs.

Elements with either of these values do not have inner or outer display types, because they don’t generate any boxes at all.

Note: As these values cause affected elements to not generate a box, anonymous box generation rules will ignore the elided elements entirely, as if they did not exist in the box tree.

Markup-based relationships, however, are not affected by these values, as they are solely rendering-time effects. For example, although they may affect which table cell appears in a column, they do not affect which table cell is associated with a particular column element. Similarly, they cannot affect which HTML summary element is associated with a particular table or whether a legend is considered to be labelling the contents of a particular fieldset.

2.6. Precomposed Inline-level Display Values

CSS level 2 used a single-keyword syntax for display, requiring separate keywords for block-level and inline-level variants of the same layout mode. These <display-legacy> keywords map as follows:

inline-block
Computes to inline flow-root.
inline-table
Computes to inline table.
inline-flex
Computes to inline flex.
inline-grid
Computes to inline grid.

Note: Although these keywords and their equivalents compute to the same value, their specified values remain distinct.

Note: The getComputedStyle() serialization rules will always output these precomposed keywords rather than the equivalent two-keyword pairs due to the shortest, most backwards-compatible serialization principle.

2.7. Automatic Box Type Transformations

Some layout effects require blockification or inlinification of the box type, which sets the box’s computed outer display type to block or inline (respectively). (This has no effect on display types that generate no box at all, such as none or contents.) Additionally:

Note: There are two methods used to fix up box types when a box is mismatched to its context. One is transformation of the computed value of display, such as blockification and inlinification described here. The other, taking place during box tree construction (after computed values have been determined), is the creation of intermediary anonymous boxes, such as happens in tables, ruby, and flow layout.

Some examples of computed-value fixup include:

The root element’s display type is always blockified. Additionally, a display of contents computes to block on the root element.

3. Display Order: the order property

order

In all current engines.

Firefox20+Safari9+Chrome29+
Opera12.1+Edge79+
Edge (Legacy)12+IE11
Firefox for Android20+iOS Safari9+Chrome for Android29+Android WebView4.4+Samsung Internet2.0+Opera Mobile12.1+
Name: order
Value: <integer>
Initial: 0
Applies to: flex items and grid items
Inherited: no
Percentages: n/a
Computed value: specified integer
Canonical order: per grammar
Animation type: by computed value type
Tests

Boxes are generally displayed and laid out in the same order as they appear in the source document. In some formatting contexts, the order property can be used to rearrange the order of boxes to deliberately create a divergence of the logical order of elements and their spatial arrangement on the 2D visual canvas. (See § 3.1 Reordering and Accessibility.)

Specifically, the order property controls the order in which flex items or grid items appear within their container, by assigning them to ordinal groups. It takes a single <integer> value, which specifies which ordinal group the item belongs to.

Here’s an example of a catalog item card which has a title, a photo, and a description. Within each entry, the source document content is ordered logically with the title first, followed by the description and the photo. This provides a sensible ordering for speech rendering and in non-CSS browsers. For a more compelling visual presentation, however, order is used to pull the image up from later in the content to the top of the card.
article.sale-item {
  display: flex;
  flex-flow: column;
}
article.sale-item > img {
  order: -1; /* Shift image before other content (in visual order) */
  align-self: center;
}
<article class="sale-item">
  <h1>Computer Starter Kit</h1>
  <p>This is the best computer money can buy, if you don’t have much money.
  <ul>
    <li>Computer
    <li>Monitor
    <li>Keyboard
    <li>Mouse
  </ul>
  <img src="images/computer.jpg"
    alt="You get: a white desktop computer with matching peripherals.">
</article>
You get: a white desktop computer with matching keyboard and monitor.

Computer Starter Kit

This is the best computer money can buy, if you don’t have much money.

  • Computer
  • Monitor
  • Keyboard
  • Mouse
An example rendering of the code above.

Flex and grid containers lay out their contents in order-modified document order, starting from the lowest numbered ordinal group and going up. Items with the same ordinal group are laid out in the order they appear in the source document. This also affects the painting order [CSS2], exactly as if the flex/grid items were reordered in the source document. Absolutely-positioned children of a flex/grid container are treated as having order: 0 for the purpose of determining their painting order relative to flex/grid items.

Unless otherwise specified by a future specification, this property has no effect on boxes that are not flex items or grid items.

3.1. Reordering and Accessibility

The order property does not affect ordering in non-visual media (such as speech). Likewise, order does not affect the default traversal order of sequential navigation modes (such as cycling through links, see e.g. tabindex [HTML]).

Authors must use order only for visual, not logical, reordering of content. Style sheets that use order to perform logical reordering are non-conforming.

Note: This is so that non-visual media and non-CSS UAs, which typically present content linearly, can rely on a logical source order, while order is used to tailor the visual order. (Since visual perception is two-dimensional and non-linear, the desired visual order is not always logical.)

In order to preserve the author’s intended ordering in all presentation modes, authoring tools—including WYSIWYG editors as well as Web-based authoring aids—must reorder the underlying document source and not use order to perform reordering unless the author has explicitly indicated that the underlying document order (which determines speech and navigation order) should be out-of-sync with the visual order.

For example, a tool might offer both drag-and-drop reordering of flex items as well as handling of media queries for alternate layouts per screen size range.

Since most of the time, reordering should affect all screen ranges as well as navigation and speech order, the tool would perform drag-and-drop reordering at the DOM layer. In some cases, however, the author may want different visual orderings per screen size. The tool could offer this functionality by using order together with media queries, but also tie the smallest screen size’s ordering to the underlying DOM order (since this is most likely to be a logical linear presentation order) while using order to determine the visual presentation order in other size ranges.

This tool would be conformant, whereas a tool that only ever used order to handle drag-and-drop reordering (however convenient it might be to implement it that way) would be non-conformant.

Note: User agents, including browsers, accessible technology, and extensions, may offer spatial navigation features. This section does not preclude respecting the order property when determining element ordering in such spatial navigation modes; indeed it would need to be considered for such a feature to work. But order is not the only (or even the primary) CSS property that would need to be considered for such a spatial navigation feature. A well-implemented spatial navigation feature would need to consider all the layout features of CSS that modify spatial relationships.

4. Invisibility: the visibility property

Name: visibility
Value: visible | hidden | collapse
Initial: visible
Applies to: all elements
Inherited: yes
Percentages: N/A
Computed value: as specified
Canonical order: per grammar
Animation type: discrete
Media: visual

The visibility property specifies whether the box is rendered. Invisible boxes still affect layout. (Set the display property to none to suppress box generation altogether.). Values have the following meanings:

visible
The generated box is visible, as normal.
hidden
Any boxes generated by the element are invisible. Descendants of the element can, however, be visible if they have visibility: visible.
collapse
Indicates that the box is collapsed, which can cause it to take up less space than otherwise in a formatting-context–specific way. See dynamic row and column effects in tables [CSS2] and collapsed flex items in flex layout [CSS-FLEXBOX-1]. In all other cases, however, (i.e. unless otherwise specified) this simply makes the box invisible, just like hidden.

Note: Currently, many user agents and/or accessibility tools don’t correctly implement the accessibility implications of visible elements with semantic relationships to invisible elements, so, for example, making parent elements with special roles (such as table rows) invisible while leaving child elements with special roles (such as table cells) visible can be problematic for users of those tools. Authors should avoid creating these situations until the tooling situation improves.

Invisible boxes are not rendered (as if they were fully transparent), cannot be interacted with (and behave as if they had pointer-events: none), are removed from navigation (similar to display: none), and are also not rendered to speech (except when speak is always [CSS-SPEECH-1]). However, as with display: contents, their semantic role as a container is not affected, to ensure that any visible descendants are properly interpreted.

Note: If speak is always, an otherwise invisible box is rendered to speech, and may be interacted with using non-visual/spatial methods.

While temporarily hiding things with display: none is often sufficient, doing so removes the elements from layout entirely, possibly causing unwanted movement or reflow of the page when an element is hidden or shown. visibility: hidden can instead be used to keep the page’s layout stable as something is hidden and displayed.

For example, here is a (deliberately simplified) possible implementation of a "spoiler" element that can be revealed by clicking on the hidden text:

<p>The symbolism earlier in the movie becomes obvious at the end,
  when it’s revealed that <spoiler-text><span>Luke is his own father</span></spoiler-text>,
  making the wizard’s cryptic riddles meaningful.
<style>
spoiler-text { border-bottom: 1px solid; }
spoiler-text > span { visibility: hidden; }
spoiler-text.shown > span { visibility: visible; }
</style>
<script>
[...document.querySelectorAll("spoiler-text")].forEach(el=>{
  el.addEventListener("click", e=>el.classList.toggle("shown"));
});
</script>

This example is deliberately significantly simplified. It is missing a number of accessiblity and UX features that a well-designed spoiler element would have to show off the visibility usage more plainly. Don’t copy this code for a real site.

5. Run-In Layout

A run-in box is a box that merges into a block that comes after it, inserting itself at the beginning of that block’s inline-level content. This is useful for formatting compact headlines, definitions, and other similar things, where the appropriate DOM structure is to have a headline preceding the following prose, but the desired display is an inline headline laying out with the text.

For example, dictionary definitions are often formatted so that the word is inline with the definition:
<dl class='dict'>
  <dt>dictionary
  <dd>a book that lists the words of a language in alphabetical
      order and gives their meaning, or that gives the equivalent
      words in a different language.
  <dt>glossary
  <dd>an alphabetical list of terms or words found in or relating
      to a specific subject, text, or dialect, with explanations; a
      brief dictionary.
</dl>
<style>
.dict > dt {
  display: run-in;
}
.dict > dt::after {
  content: ": "
}
</style>

Which is formatted as:

dictionary: a book that lists the words of a language
in alphabetical order and explains their meaning.

glossary: an alphabetical list of terms or words found
in or relating to a specific subject, text, or dialect,
with explanations; a brief dictionary.

A run-in box behaves exactly as any other inline-level box, except:

A run-in sequence is a maximal sequence of consecutive sibling run-in boxes and intervening white space and/or out-of-flow boxes.

Note: This statement implies that out-of-flow boxes are reparented if they are between two run-in boxes. Another alternative would be to leave behind the intervening out-of-flow boxes, or to have out-of-flow boxes impede the running-in of earlier boxes. Implementers and authors are encouraged to contact the CSSWG if they have a preferred behavior, as this one was picked somewhat at random.

This fixup occurs before the anonymous block and inline box fixup described in CSS2§9.2, and affects the determination of the first formatted line of the affected elements as if the run-in sequence were originally in its final location in the box tree.

Note: As the earliest run-in represents the first text on the first formatted line of its containing block, a ::first-letter pseudo-element applied to that block element selects the first letter of the run-in, rather than the first letter of its own contents.

Note: This run-in model is slightly different from the one proposed in earlier revisions of [CSS2].

Appendix A: Glossary

The following terms are defined here for convenience:

principal box
When an element generates one or more boxes, one of them is the principal box, which contains its descendant boxes and generated content, and is also the box involved in any positioning scheme.

Some elements may generate additional boxes in addition to the principal box (such as list-item elements, which generate an additional marker box, or table elements, which generate a principal table wrapper box and an additional table grid box). These additional boxes are placed with respect to the principal box.

inline-level
Content that participates in inline layout. Specifically, inline-level boxes and text runs.
block-level
Content that participates in block layout. Specifically, block-level boxes.
inline box
A non-replaced inline-level box whose inner display type is flow. The contents of an inline box participate in the same inline formatting context as the inline box itself.
inline
Used as a shorthand for inline box or inline-level box where unambiguous, or as an adjective meaning inline-level. The latter usage is deprecated.
atomic inline
An inline-level box that is replaced (such as an image) or that establishes a new formatting context (such as an inline-block or inline-table) and cannot split across lines (as inline boxes and ruby containers can).

Any inline-level box whose inner display type is not flow establishes a new formatting context of the specified inner display type.

block container
A block container either contains only inline-level boxes participating in an inline formatting context, or contains only block-level boxes participating in a block formatting context (possibly generating anonymous block boxes to ensure this constraint, as defined in CSS2§9.2.1.1).

A block container that contains only inline-level content establishes a new inline formatting context. The element then also generates a root inline box which wraps all of its inline content. Note, this root inline box concept effectively replaces the "anonymous inline element" concept introduced in CSS2§9.2.2.1.

A block container establishes a new block formatting context if its parent formatting context is not a block formatting context; otherwise, when participating in a block formatting context itself, it either establishes a new block formatting context for its contents or continues the one in which it participates, as determined by the constraints of other properties (such as overflow or align-content).

Note: A block container box can both establish a block formatting context and an inline formatting context simultaneously.

block box
A block-level box that is also a block container.

Note: Not all block container boxes are block-level boxes: non-replaced inline blocks and non-replaced table cells, for example, are block containers but not block-level boxes. Similarly, not all block-level boxes are block containers: block-level replaced elements (display: block) and flex containers (display: flex), for example, are not block containers.

block
Used as a shorthand for block box, block-level box, or block container box, where unambiguous.
replaced element
An element whose content is outside the scope of the CSS formatting model, such as an image or embedded document. For example, the content of the HTML img element is often replaced by the image that its src attribute designates.

Replaced elements often have natural dimensions. For example, a bitmap image has a natural width and a natural height specified in absolute units (from which the natural ratio can obviously be determined). On the other hand, other objects may not have any natural dimensions (for example, a blank HTML document). See [css-images-3].

User agents may consider a replaced element to not have any natural dimensions if it is believed that those dimensions could leak sensitive information to a third party. For example, if an HTML document changed natural size depending on the user’s bank balance, then the UA might want to act as if that resource had no natural dimensions.

The content of replaced elements is not considered in the CSS formatting model; however, their natural dimensions are used in various layout calculations. Replaced elements always establish an independent formatting context.

A non-replaced element is one that is not replaced, i.e. whose rendering is dictated by the CSS model.

containing block
A rectangle that forms the basis of sizing and positioning for the boxes associated with it. Notably, a containing block is not a box (it is a rectangle), however it is often derived from the dimensions of a box. Each box is given a position with respect to its containing block, but it is not confined by this containing block; it can overflow. The phrase “a box’s containing block” means “the containing block in which the box lives,” not the one it generates.

In general, the edges of a box act as the containing block for descendant boxes; we say that a box “establishes” the containing block for its descendants. If properties of a containing block are referenced, they reference the values on the box that generated the containing block. (For the initial containing block, values are taken from the root element unless otherwise specified.)

See [CSS2] Section 9.1.2 and Section 10.1 and CSS Positioned Layout 3 § 2.1 Containing Blocks of Positioned Boxes for details.

containing block chain
A sequence of successive containing blocks that form an ancestor-descendant chain through the containing block relation. For example, an inline box’s containing block is the content box of its closest block container ancestor; if that block container is an in-flow block, then its containing block is formed by its parent block container; if that grandparent block container is absolutely positioned, then its containing block is the padding edges of its closest positioned ancestor (not necessarily its parent), and so on up to the initial containing block.
initial containing block
The containing block of the root element. See CSS2.1§10.1 for continuous media; and [CSS-PAGE-3] for paged media.
formatting context
A formatting context is the environment into which a set of related boxes are laid out. Different formatting contexts lay out their boxes according to different rules. For example, a flex formatting context lays out boxes according to the flex layout rules [CSS-FLEXBOX-1], whereas a block formatting context lays out boxes according to the block-and-inline layout rules [CSS2]. Additionally, some types of formatting contexts interleave and co-exist: for example, an inline formatting context exists within and interacts with the block formatting context of the element that establishes it, and a ruby container overlays a ruby formatting context over the inline formatting context in which its ruby base container participates.

A box either establishes a new independent formatting context or continues the formatting context of its containing block. In some cases, it might additionally establish a new (non-independent) co-existing formatting context. Unless otherwise specified, however, establishing a new formatting context creates an independent formatting context. The type of formatting context established by the box is determined by its inner display type. E.g. a grid container establishes a new grid formatting context, a ruby container establishes a new ruby formatting context, and a block container can establish a new block formatting context and/or a new inline formatting context. See the display property.

independent formatting context
When a box establishes an independent formatting context (whether that formatting context is of the same type as its parent or not), it essentially creates a new, independent layout environment: except through the sizing of the box itself, the layout of its descendants is (generally) not affected by the rules and contents of the formatting context outside the box, and vice versa.

For example, in a block formatting context, floated boxes affect the layout of surrounding boxes. But their effects do not escape their formatting context: the box establishing their formatting context grows to fully contain them, and floats from outside that box are not allowed to protrude into and affect the contents inside the box.

As another example, margins do not collapse across formatting context boundaries.

Exclusions are able to affect content across independent formatting context boundaries. (At time of writing, they are the only layout feature that can.) [CSS3-EXCLUSIONS]

Certain properties can force a box to establish an independent formatting context in cases where it wouldn’t ordinarily. For example, making a box out-of-flow causes it to blockify as well as to establish an independent formatting context. As another example, certain values of the contain property can cause a box to establish an independent formatting context. Turning a block into a scroll container will cause it to establish an independent formatting context; however turning a subgrid into a scroll container will not—it continues to act as a subgrid, with its contents participating in the layout of its parent grid container.

A block box that establishes an independent formatting context establishes a new block formatting context for its contents. In most other cases, forcing a box to establish an independent formatting context is a no-op—either the box already establishes an independent formatting context (e.g. flex containers), or it’s not possible to establish a totally independent new formatting context on that type of box (e.g. non-replaced inline boxes).

block formatting context
inline formatting context
Block and inline formatting contexts are defined in CSS 2.1 Section 9.4. Inline formatting contexts exist within (are part of their containing) block formatting contexts; for example, line boxes belonging to the inline formatting context interact with floats belonging to the block formatting context.
block layout
The layout of block-level boxes, performed within a block formatting context.
block formatting context root
A block container that establishes a new block formatting context.
BFC
Abbreviation for block formatting context or block formatting context root. Has various informal definitions referring to boxes which contain internal floats, exclude external floats, and suppress margin collapsing, and may therefore refer specifically to one of:
out-of-flow
in-flow
A box is out-of-flow if it is extracted from its expected position and interaction with surrounding content and laid out using a different paradigm outside the normal flow of content in its parent formatting context. This occurs if the box is floated (via float) or absolutely positioned (via position). A box is in-flow if it is not out-of-flow.

Note: Some formatting contexts inhibit floating, so that an element with float: left is not necessarily out-of-flow.

document order
The order in which boxes or content occurs in the document (which can be different from the order in which it appears for rendering). For the purpose of determining the relative order of pseudo-elements, the box-tree order is used, see CSS Pseudo-Elements 4 § 4 Tree-Abiding Pseudo-elements.

See [CSS2] Chapter 9 for a fuller definition of these terms.

Appendix B: Effects of display: contents on Unusual Elements

This section is (currently) non-normative.

Some elements aren’t rendered purely by CSS box concepts; for example, replaced elements (such as img), many form controls (such as input), and SVG elements.

This appendix defines how they interact with display: contents.

HTML Elements

br
wbr
meter
progress
canvas
embed
object
audio
iframe
img
video
frame
frameset
input
textarea
select

display: contents computes to display: none.

legend

Per HTML, a legend with display: contents is not a rendered legend, so it does not have magical display behavior. (Thus, it reacts to display: contents normally.)

button
details
fieldset

These elements don’t have any special behavior; display: contents simply removes their principal box, and their contents render as normal.

any other HTML element

Behaves as normal for display: contents.

SVG Elements

An svg element that has CSS box layout (this includes all svg whose parent is an HTML element, as well as document root elements)

display: contents computes to display: none.

All other SVG container elements that are also renderable elements
SVG text content child elements
use

display: contents strips the element from the formatting tree, and hoists its contents up to display in its place. These contents include the shadow-DOM content for use.

any other SVG elements

display: contents computes to display: none.

The intention here is that the display: none behavior applies whenever the "rendering context" inside the element is different than the context outside of it. If the element’s child elements would not be valid children of the element’s parent, you cannot simply hoist them up the formatting tree.

For example, text content and text formatting elements in SVG require a text element context; if you remove a text, its child text content and elements are no longer valid. For that reason, display: contents on text prevents the entire text element from being rendered. In contrast, any valid content inside a tspan or textPath is also valid content inside the parent text formatting context, so the hoisting behavior applies for these elements.

Similarly, if hoisting would convert the children from non-rendered elements (e.g., a shape inside a pattern or symbol) to rendered elements (e.g., a shape that is a direct child of the svg), that is an invalid change of rendering context. Never-rendered container elements therefore cannot be "un-boxed" by display: contents.

When an element is stripped from the formatting tree, then any SVG attributes on that element that control layout and visual formatting are ignored when rendering the contents. However, SVG presentation attributeswhich map to CSS properties—continue to affect value processing and inheritance [CSS-CASCADE-3]; thus such attributes can affect the layout and visual formatting of the element’s descendants by influencing the values of such properties on those descendants.

MathML Elements

For all MathML elements, display: contents computes to display: none.

Appendix C: Box Construction Guidelines for Spec Authors

This section is non-normative guidance for specification authors.

Acknowledgments

We would like to thank the many people who have attempted to separate out the disparate details of box generation over the years, most particularly Bert Bos, whose last attempt with display-model and display-role didn’t get anywhere, but primed us for the current spec; Anton Prowse, whose relentless assault on CSS2.1 Chapter 9 forced some order out of the chaos; and Oriol Brufau, who teased apart dozens of fine distinctions and errors in this spec. Honorable mentions also go to David Baron, Mats Palmgren, Ilya Streltsyn, and Boris Zbarsky for their feedback.

Changes

Changes Since 2020 Candidate Recommendation

A Disposition of Comments since the 19 May 2020 Candidate Recommendation is available.

Changes since the 18 December 2020 Candidate Recommendation Draft include:

Previous changes since the 19 May 2020 Candidate Recommendation consist only of minor editorial clarifications and cross-linking improvements.

Changes Since 2019 Candidate Recommendation

Changes since the 11 July 2019 Candidate Recommendation include:

Changes since the 28 August 2018 Candidate Recommendation include:

Changes Prior to Candidate Recommendation Status

A Disposition of Comments is available.

Changes since the 20 April 2018 Working Draft include:

Changes since the 20 July 2017 Working Draft include:

Changes since the 26 January 2017 Working Draft include:

A Disposition of Comments is also available.

Changes since the 15 October 2015 Working Draft include:

Changes since the 21 July 2015 Working Draft include:

Changes since the 11 September 2014 Working Draft include:

Privacy Considerations

This specification introduces no new privacy considerations.

Security Considerations

This specification introduces no new security considerations.

Self-Review Questionaire

Per the Self-Review Questionnaire: Security and Privacy: Questions to Consider

  1. Does this specification deal with personally-identifiable information?

    No.

  2. Does this specification deal with high-value data?

    No.

  3. Does this specification introduce new state for an origin that persists across browsing sessions?

    No.

  4. Does this specification expose persistent, cross-origin state to the web?

    No.

  5. Does this specification expose any other data to an origin that it doesn’t currently have access to?

    No.

  6. Does this specification enable new script execution/loading mechanisms?

    No.

  7. Does this specification allow an origin access to a user’s location?

    No.

  8. Does this specification allow an origin access to sensors on a user’s device?

    No.

  9. Does this specification allow an origin access to aspects of a user’s local computing environment?

    No.

  10. Does this specification allow an origin access to other devices?

    No.

  11. Does this specification allow an origin some measure of control over a user agent’s native UI?

    No.

  12. Does this specification expose temporary identifiers to the web?

    No.

  13. Does this specification distinguish between behavior in first-party and third-party contexts?

    No.

  14. How should this specification work in the context of a user agent’s "incognito" mode?

    No differently.

  15. Does this specification persist data to a user’s local device?

    No.

  16. Does this specification have a "Security Considerations" and "Privacy Considerations" section?

    Yes.

  17. Does this specification allow downgrading default security characteristics?

    No.

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-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 21 April 2020. WD. URL: https://www.w3.org/TR/css-align-3/
[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BOX-4]
Elika Etemad. CSS Box Model Module Level 4. 21 April 2020. WD. URL: https://www.w3.org/TR/css-box-4/
[CSS-BREAK-3]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 4 December 2018. CR. URL: https://www.w3.org/TR/css-break-3/
[CSS-BREAK-4]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 4. 18 December 2018. WD. URL: https://www.w3.org/TR/css-break-4/
[CSS-CASCADE-3]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 3. 11 February 2021. REC. URL: https://www.w3.org/TR/css-cascade-3/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 29 August 2021. WD. URL: https://www.w3.org/TR/css-cascade-5/
[CSS-CONTAIN-2]
Tab Atkins Jr.; Florian Rivoal; Vladimir Levin. CSS Containment Module Level 2. 16 December 2020. WD. URL: https://www.w3.org/TR/css-contain-2/
[CSS-FLEXBOX-1]
Tab Atkins Jr.; et al. CSS Flexible Box Layout Module Level 1. 19 November 2018. CR. URL: https://www.w3.org/TR/css-flexbox-1/
[CSS-GRID-1]
Tab Atkins Jr.; et al. CSS Grid Layout Module Level 1. 18 December 2020. CR. URL: https://www.w3.org/TR/css-grid-1/
[CSS-GRID-2]
Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Grid Layout Module Level 2. 18 December 2020. CR. URL: https://www.w3.org/TR/css-grid-2/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. 17 December 2020. CR. URL: https://www.w3.org/TR/css-images-3/
[CSS-INLINE-3]
Dave Cramer; Elika Etemad; Steve Zilles. CSS Inline Layout Module Level 3. 27 August 2020. WD. URL: https://www.w3.org/TR/css-inline-3/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 3 June 2020. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-PAGE-3]
Elika Etemad; Simon Sapin. CSS Paged Media Module Level 3. 18 October 2018. WD. URL: https://www.w3.org/TR/css-page-3/
[CSS-POSITION-3]
Elika Etemad; et al. CSS Positioned Layout Module Level 3. 19 May 2020. WD. URL: https://www.w3.org/TR/css-position-3/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 31 December 2020. WD. URL: https://www.w3.org/TR/css-pseudo-4/
[CSS-RUBY-1]
Elika Etemad; et al. CSS Ruby Annotation Layout Module Level 1. 10 March 2021. WD. URL: https://www.w3.org/TR/css-ruby-1/
[CSS-SPEECH-1]
Daniel Weck. CSS Speech Module. 10 March 2020. CR. URL: https://www.w3.org/TR/css-speech-1/
[CSS-TABLES-3]
François Remy; Greg Whitworth; David Baron. CSS Table Module Level 3. 27 July 2019. WD. URL: https://www.w3.org/TR/css-tables-3/
[CSS-TEXT-3]
Elika Etemad; Koji Ishii; Florian Rivoal. CSS Text Module Level 3. 22 April 2021. CR. URL: https://www.w3.org/TR/css-text-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 15 July 2021. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
[CSSOM]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). 26 August 2021. WD. URL: https://www.w3.org/TR/cssom-1/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[MEDIAQUERIES-5]
Dean Jackson; Florian Rivoal; Tab Atkins Jr.. Media Queries Level 5. 31 July 2020. WD. URL: https://www.w3.org/TR/mediaqueries-5/
[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
[SELECTORS-4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 21 November 2018. WD. URL: https://www.w3.org/TR/selectors-4/
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. 4 October 2018. CR. URL: https://www.w3.org/TR/SVG2/

Informative References

[CSS3-EXCLUSIONS]
Rossen Atanassov; Vincent Hardy; Alan Stearns. CSS Exclusions Module Level 1. 15 January 2015. WD. URL: https://www.w3.org/TR/css3-exclusions/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value Media
display [ <display-outside> || <display-inside> ] | <display-listitem> | <display-internal> | <display-box> | <display-legacy> inline all elements no n/a not animatable per grammar a pair of keywords representing the inner and outer display types plus optional list-item flag, or a <display-internal> or <display-box> keyword; see prose in a variety of specs for computation rules
order <integer> 0 flex items and grid items no n/a by computed value type per grammar specified integer
visibility visible | hidden | collapse visible all elements yes N/A discrete per grammar as specified visual