CSS Display Module Level 3

Abstract

Status of this document

Table of Contents

1 Introduction

This section is not normative.

The display property, introduced in CSS 2.1, defines what kind of boxes an element generates (and whether it generates boxes at all), and how it lays out its contents.

These concepts are actually rather independent, though they’re conflated by the display property. This causes some pain when a property value intended to affect one aspect (such as setting an element to display:none to suppress box generation) affects another aspect (such as losing the memory of what it was before display:none, so that it can be set back to that value later).

This specification subsumes the CSS 2.1 definition of the display property, and redefines it to be a shorthand property for a small family of longhands, each controlling an independent aspect of an element’s "display".

1.1 Module interactions

This specification transforms the display property into a shorthand property, and defines several longhand properties that it expands into or effects.

This module replaces and extends the definition of the display property defined in [CSS21] section 9.2.4.

None of the 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.

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 it has not been repeated explicitly.

2 Controlling Layout Modes

The display shorthand and its associated family of properties control the layout mode of elements (how the element determines the sizes and positions of itself and its descendants), and what boxes they and their descendants generate.

2.1 Telling Contents How to Lay Out: the display-inside property

The display-inside property specifies the inner display type of the box generated by the element, dictating how its contents lay out inside the box.

auto

If the element’s computed display-outside value is inline-level, the element is an inline element, and lays out its contents using inline layout. [CSS21] If the element’s computed display-outside value is an layout-specific internal type, this elements acts as normal for its given display-outside value. Otherwise, this value computes to block.

block

The element lays out its contents using block layout. [CSS21]

table

The element lays out its contents using table layout. [CSS21]

flex

The element lays out its contents using flex layout. [CSS3-FLEXBOX]

grid

The element lays out its contents using grid layout. [CSS3-GRID-LAYOUT]

ruby

The element lays out its contents using ruby layout. [CSS3RUBY]

2.2 Interacting with Ancestors and Siblings: the display-outside property

The display-outside property specifies the outer display type of the box generated by the element, dictating how the element participates in its parent formatting context.

block-level

The element generates a block-level box, and participates in a block formatting context. Other formatting contexts, such as flex formatting contexts, may also work with block-level elements. [CSS21]

inline-level

The element generates an inline-level box, and participates in an inline formatting context. [CSS21]

run-in

The element generates a run-in box. Run-in elements act like inlines or blocks, depending on the surrounding elements. See §4 Run-In Layout for details.

contents

The element itself does not generate any boxes, but its children and pseudo-elements still generate boxes as normal. For the purposes of box generation and layout, the element must be treated as if it had been replaced with its children and pseudo-elements in the document tree.

contents currently only has an effect on box generation and layout. Other things that care about the document tree are unaffected, like counter scopes. Is this what we want?

none

The element generates no boxes, and does not participate in any formatting context.

Note: This value exists for legacy reasons, and interacts with the separate box-suppress property.

It is recommended that box-suppress be used instead of display: none, so that the element’s display type is automatically preserved for when it’s no longer suppressed.

table-row-group, table-header-group, table-footer-group, table-row, table-cell, table-column-group, table-column, table-caption

The element is an internal table element, and participates in a table layout context. [CSS21]

table-cell and table-caption are layout-specific leaf types; the rest are layout-specific internal types.

ruby-base, ruby-text, ruby-base-container, ruby-text-container

The element is an internal ruby element, and participates in a ruby layout context. [CSS3RUBY]

ruby-base and ruby-text are layout-specific leaf types; ruby-base-container and ruby-text-container are layout-specific internal types.

Some values of display-outside are specialized for particular formatting contexts, and don’t have meaning outside of those specific contexts:

layout-specific internal types

These display types require their parent and children to be of particular display types. For example, a table-row box requires its parent to be a table row group box and its children to be table-cell boxes.

layout-specific leaf types

These display types require their parent to be of a particular display type, but can accept any display-inside value. For example, a table-caption box must have a table parent, but can establish any kind of formatting context for its children.

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

2.3 Creating List Markers: the display-list property

list-item

The element generates a ::marker pseudo-element and is considered a list item.

2.4 The display shorthand property

The display shorthand property sets all the display type properties in one declaration: display-outside, display-inside, and display-list. If only one keyword is specified or only one keyword is specified together with list-item, the shorthand expands as specified below:

* For list-item and inline-list-item, additionally set display-list to list-item.

2.5 Automatic Box Type Transformations

Some layout effects require blockification or inlinification of the box type, causing the box’s display-outside property, if it is not none or contents, to compute to block-level or inline-level (respectively).

When a box is inlinified, it recursively inlinifies all of its in-flow children unless it itself establishes a new formatting context, so that no block-level descendants break up the inline formatting context in which it participates.

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

3 Controlling box generation: the box-suppress property

If the computed value of display-outside is none, the computed value of box-suppress is discard. Otherwise, the computed value is the specified value.

show

The element generates boxes as normal, per its display-* properties.

discard

The element generates no boxes at all.

hide

The element generates boxes as normal, but those boxes do not participate in layout in any way, and must not be displayed.

For the purpose of any layout-related information, such as querying for the computed value of the element’s width property, it must be treated as if it did not generate any boxes.

Properties that rely on boxes but do not rely on layout, such as animations, counter-increment, etc., must work as normal on this element and its descendants.

This needs more clarity about what "layout-related" and "participates in layout" means. Does the box still generate anonymous boxes, etc.?

How does this affect speech? Is that "layout"?

We welcome better naming suggestions on this property.

4 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.

<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>

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 an inline-level box, except:

• If a run-in sequence is immediately followed by a block box that does not establish a new formatting context, it is inserted as direct children of the block box after its ::marker pseudo-element’s boxes (if any), but preceding any other boxes generates by the contents of the block (including the box generated by the ::before pseudo-element, if any).

  The reparented content is then formatted as if originally parented there. Note that only layout is affected, not inheritance, because property inheritance for non-anonymous boxes is based only on the element tree.

• Otherwise, an anonymous block box is generated around the run-in sequence and all immediately following inline-level content (up to, but not including, the next run-in sequence, if any).
• A run-in box with display-inside: auto inlinifies its contents.

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

Should out-of-flow elements get reparented, left behind, or break apart the sequence? See thread.

Note: This run-in model is slightly different from the one proposed in earlier revisions of [CSS21].

5 Glossary

The following terms are defined here for convenience:

inline-level

Content that participates in inline layout. Specifically, inline-level boxes and text.

block-level

Content that participates in block layout. Specifically, block-level boxes.

inline

A non-replaced inline-level box whose display-inside is auto. The contents of an inline box participate in the same inline formatting context as the inline box itself.

atomic inline

An inline-level box that is replaced or that establishes a new formatting context. (An inline-level box whose display-inside is not auto establishes a new formatting context of the specified type.)

block container

A box whose contents participate in a block formatting context, i.e. one whose computed display-inside value is block.

block box

A block-level box that is a block container.

block

Used as a shorthand for block box, block-level box, or block container box, where unambiguous.

containing block

A rectangle that forms the basis of sizing and positioning for the boxes associated with it (usually the children of the box that generated it). Notably, a containing block is not a box (it is a rectangle), however it is often derived from the dimensions of a box. 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, the values are taken from the root element.) See [CSS21] Section 9.1.2 and Section 10.1 for details.

initial containing block

The containing block of the root element. See CSS2.1§10.1 for continuous media; and [CSS3PAGE] for paged media.

formatting context

The thing that makes CSS do the layouts.

wordsmith this.

Block and inline formatting contexts are defined in CSS 2.1 Section 9.4.

See [CSS21] Chapter 9 for a fuller definition of these terms.

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, and Anton Prowse, whose relentless assault on CSS2.1 Chapter 9 forced some order out of the chaos.

We would also like to thank the many JavaScript libraries such as jQuery which have hacked around the "what display should I give it when you call .show()?" problem, making it extremely clear that something needed to be done on our part.

Changes

Changes since the 20 Feburary 2014 Working Draft include:

• Renamed display-box to box-suppress and display-extras to display-list.
• Moved the contents value from box-suppress to display-outside.
• Added hide value to box-suppress.
• Added blockification and inlinification rules.
• Added display: run-in and defined its layout in §4 Run-In Layout.
• Various editorial improvements.
• Added §5 Glossary, incorporating key CSS2.1 terms.

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:

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.

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.

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.

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.

References

Normative References

Informative References

Index

Property index

Issues Index