CSS Multi-column Layout Module

Abstract

Status of this document

Table of Contents

1 Introduction

(This section is not normative.)

This module describes multi-column layout in CSS. By using functionality described in this document, style sheets can declare that the content of an element is to be laid out in multiple columns.

On the Web, tables have also been used to describe multi-column layouts. The main benefit of using CSS-based columns is flexibility; content can flow from one column to another, and the number of columns can vary depending on the size of the viewport. Removing presentation table markup from documents allows them to more easily be presented on various output devices including speech synthesizers and small mobile devices.

Multi-column layouts are easy to describe in CSS. Here is a simple example:

body { column-width: 12em }

The number of columns can also be set explicitly in the style sheet:

body { column-count: 2 }

The shorthand columns property can be used to set either, or both, properties in one declaration.

body { columns: 2 }
body { columns: 12em }
body { columns: 2 12em }

Another group of properties introduced in this module describe gaps and rules between columns.

body {
  column-gap: 1em;
  column-rule: thin solid black;
}

The values of the column-rule property are similar to those of the CSS border properties. Like border, column-rule is a shorthand property.

body {
  column-gap: 1em;
  column-rule-width: thin;
  column-rule-style: solid;
  column-rule-color: black;
}

The column-fill and column-span properties give style sheets a wider range of visual expressions in multi-column layouts.

div { column-fill: balance }
h2 { column-span: all }

This specification introduces ten new properties, all of which are used in the examples above.

To indicate where column breaks should (or should not) appear, new keyword values are introduced.

h1 {
  break-before: column;
  break-inside: avoid-column;
  break-after: avoid-column;
}

If all column properties have their initial value, the layout of an element will be identical to a multi-column layout with only one column.

2 The multi-column model

A multi-column element (or multicol element for short) is an element whose column-width or column-count property is not auto and therefore acts as a container for multi-column layout.

In the traditional CSS box model, the content of an element is flowed into the content box of the corresponding element. Multi-column layout introduces a new type of container between the content box and the content, namely the column box (or column for short). The content of a multicol element is flowed into its column boxes.

Column boxes in a multi-column element are arranged into rows. Like table cells, the column boxes in a row are ordered in the inline direction of the multicol element. The column width is the length of the column box in the inline direction. The column height is the length of the column box in the block direction. All column boxes in a row have the same column width, and all column boxes in a row have the same column height. Within each row in the multi-column element, adjacent column boxes are separated by a column gap, which may contain a column rule. All column gaps in the same row are equal. All column rules in the same row are also equal, if they appear; column rules only appear between columns that both have content.

In the simplest case a multicol element will contain only one row of columns, and the height of each column will be equivalent to the used height of the multi-column element’s content box.

If the multi-column element is paginated, the height of each row is constrained by the page and the content continues in a new row of column boxes on the next page; a column box never splits across pages.

The same effect occurs when a spanning element divides the multi-column element: the columns before the spanning element are balanced and shortened to fit their content. Content after the spanning element then flows into a new row of column boxes.

Column boxes are block container boxes. That is, column boxes behave like block-level, table cell, and inline-block boxes as per CSS 2.1, section 10.1, item 2 [CSS21]. However, column boxes do not establish block container boxes for elements with ''position: fixed or position: absolute''.

img { display: block; width: 100% }

Floats that appear inside multi-column layouts are positioned with regard to the column box where the float appears.

img { display: block; float: right }

A multi-column element establishes a new block formatting context, as per CSS 2.1 section 9.4.1.

Nested multi-column elements are allowed, but there may be implementation-specific limits.

Note: It is not possible to set properties/values on column boxes. For example, the background of a certain column box cannot be set and a column box has no concept of padding, margin or borders. Future specifications may add additional functionality. For example, columns of different widths and different backgrounds may be supported.

Note: Multicol elements with column heights larger than the viewport may pose accessibility issues.

3 The number and width of columns

Finding the number and width of columns is fundamental when laying out multi-column content. These properties are used to set the number and width of columns:

• column-count
• column-width

A third property, columns, is a shorthand property which sets both column-width and column-count.

Other factors, such as explicit column breaks, content, and height constraints, may influence the actual number and width of columns.

3.1 column-width

This property describes the width of columns in multicol elements.

auto

means that the column width will be determined by other properties (e.g., column-count, if it has a non-auto value).

<length>

describes the optimal column width. The actual column width may be wider (to fill the available space), or narrower (only if the available space is smaller than the specified column width). Specified values must be greater than 0.

div {
  width: 100px;
  column-width: 45px;
  column-gap: 0;
  column-rule: none;
}

div {
  width: 40px;
  column-width: 45px;
  column-gap: 0;
  column-rule: none;
}

To ensure that column-width can be used with vertical text, column width means the length of the line boxes inside the columns.

Note: The reason for making column-width somewhat flexible is to achieve scalable designs that can fit many screen sizes. To set an exact column width, the column gap and the width of the multicol element (assuming horizontal text) must also be specified.

3.2 column-count

This property describes the number of columns of a multicol element.

auto

means that the number of columns will be determined by other properties (e.g., column-width, if it has a non-auto value).

<integer>

describes the optimal number of columns into which the content of the element will be flowed. Values must be greater than 0. If both column-width and column-count have non-auto values, the integer value describes the maximum number of columns.

body { column-count: 3 }

3.3 columns

This is a shorthand property for setting column-width and column-count. Omitted values are set to their initial values.

columns: 12em;      /* column-width: 12em; column-count: auto */
columns: auto 12em; /* column-width: 12em; column-count: auto */
columns: 2;         /* column-width: auto; column-count: 2 */
columns: 2 auto;    /* column-width: auto; column-count: 2 */
columns: auto;      /* column-width: auto; column-count: auto */
columns: auto auto; /* column-width: auto; column-count: auto */

3.4 Pseudo-algorithm

The pseudo-algorithm below determines the used values for column-count (N) and column-width (W) when . There is one other variable in the pseudo-algorithm: U is the used width of the multi-column element.

Note: The used width U of the multi-column element can depend on the element’s contents, in which case it also depends on the computed values of the column-count and column-width properties. This specification does not define how U is calculated. Another module (probably the Basic Box Model [CSS3BOX] or the Intrinsic & Extrinsic Sizing Module [CSS3-SIZING]) is expected to define this.

The floor(X) function returns the largest integer Y ≤ X.

(01)  if ((column-width = auto) and (column-count = auto)) then
(02)      exit; /* not a multicol element */
(03)  if column-width = auto then
(04)      N := column-count
(05)  else if column-count = auto then
(06)      N := max(1,
(07)        floor((U + column-gap)/(column-width + column-gap)))
(08)  else
(09)      N := min(column-count, max(1,
(10)        floor((U + column-gap)/(column-width + column-gap))))

And:

(11)  W := max(0, ((U + column-gap)/N - column-gap))

In paged media, user agents may perform this calculation on a per-page basis.

The used value for column-count is calculated without regard for explicit column breaks or constrained column heights, while the actual value takes these into consideration.

div { width: 40em; columns: 20em; column-gap: 0 }
p { break-after: column }

<div>
  <p>one
  <p>two
  <p>three
</div>

div {
  width: 80em;
  height: 10em;
  columns: 20em;
  column-gap: 0;
  column-fill: auto;
}

<div>foo</div>

3.5 Stacking context

All column boxes in a multi-column element are in the same stacking context and the drawing order of their contents is as specified in CSS 2.1. Column boxes do not establish new stacking contexts.

4 Column gaps and rules

Column gaps and rules are placed between columns in the same multicol element. The length of the column gaps and column rules is equal to the column height. Column gaps take up space. That is, column gaps will push apart content in adjacent columns (within the same multicol element).

A column rule is drawn in the middle of the column gap with the endpoints at opposing content edges of the multicol element. Column rules do not take up space. That is, the presence or thickness of a column rule will not alter the placement of anything else. If a column rule is wider than its gap, the adjacent column boxes will overlap the rule, and the rule may possibly extend outside the box of the multicol element. Column rules are painted in the inline content layer, but below all inline content inside the multicol element. Column rules are only drawn between two columns that both have content.

4.1 column-gap

<length>

Specifies the gap between columns. If there is a column rule between columns, it will appear in the middle of the gap.

The <length> cannot be negative.

normal

Identical to <length>, but with a UA-specified length. A value of 1em is suggested.

4.2 column-rule-color

<color>

Specifies the color of the column rule.

4.3 column-rule-style

The column-rule-style property sets the style of the rule between columns of an element. The <‘border-style’> values are interpreted as in the collapsing border model.

The none value forces the computed value of column-rule-width to be 0.

4.4 column-rule-width

This property sets the width of the rule between columns. Negative values are not allowed.

4.5 column-rule

This property is a shorthand for setting column-rule-width, column-rule-style, and column-rule-color at the same place in the style sheet. Omitted values are set to their initial values.

body {
  column-gap: 1em;
  column-rule-width: 1em;
  column-rule-style: solid;
  column-rule-color: black;
}

5 Column breaks

When content is laid out in multiple columns, the user agent must determine where column breaks are placed. The problem of breaking content into columns is similar to breaking content into pages, which is described in CSS 2.1, section 13.3.3 [CSS21].

Three new properties are introduced to allow column breaks to be described in the same properties as page breaks: break-before, break-after, and break-inside. These properties take the same values as page-break-before, page-break-after, and page-break-inside [CSS21]. In addition, some new keyword values are added.

5.1 break-before, break-after, break-inside

These properties describe page/column break behavior before/after/inside the generated box. These values are normatively defined in [CSS21]:

auto

Neither force nor forbid a page/column break before (after, inside) the generated box.

always

Always force a page break before (after) the generated box.

avoid

Avoid a page/column break before (after, inside) the generated box.

left

Force one or two page breaks before (after) the generated box so that the next page is formatted as a left page.

right

Force one or two page breaks before (after) the generated box so that the next page is formatted as a right page.

This specification adds the following new values:

page

Always force a page break before (after) the generated box.

column

Always force a column break before (after) the generated box.

avoid-page

Avoid a page break before (after, inside) the generated box.

avoid-column

Avoid a column break before (after, inside) the generated box.

When a page or column break splits a box, the box’s margins, borders, and padding have no visual effect where the split occurs. However, the margin immediately after a forced page/column break will be preserved. A forced page/column break is a break that does not occur naturally.

Note: In the future, new properties may describe alternate ways to handle margins, borders and padding around page/column breaks.

.multicol { column-width: 8em }
.multicol h2 { break-before: column; margin-top: 2em }
.multicol img { break-after: column }

p { break-inside: avoid-column }

6 Spanning columns

The column-span property makes it possible for an element to span across several columns.

6.1 column-span

This property describes how many columns an element spans across. Values are:

none

The element does not span multiple columns.

all

The element spans across all columns of the nearest multicol ancestor in the same block formatting context. The element spans across all columns. Content in the normal flow that appears before the element is automatically balanced across all columns before the element appears. The element establishes a new block formatting context.

An element that spans more than one column is called a spanning element and the box it creates is called a spanner.

h2 { column-span: all; background: silver }

A spanning element takes up more space than the element would take up otherwise. When space is limited, it may be impossible to find room for the spanning element. In these cases, user agents may treat the element as if none had been specified on this property.

h2 {
  margin: 0.5em 0;
  column-span: all;
  background: silver
}
p { margin-top: 1em }

7 Filling columns

There are two strategies for filling columns: columns can either be balanced, or not. If columns are balanced, user agents should try to minimize variations in column height, while honoring forced breaks, widows and orphans, and other properties that may affect column heights. If columns are not balanced, they are filled sequentially; some columns may end up partially filled, or with no content at all.

7.1 column-fill

The values are:

balance

Balance content equally between columns, as far as possible. In paged media, only the last page is balanced.

balance-all

Balance content equally between columns, as far as possible. In paged media, all pages are balanced.

auto

fill columns sequentially

In continuous media, this property does not have any effect in overflow columns.

article { width: 60em; height: auto; columns: 4; column-fill: balance }

article { width: 60em; height: auto; columns: 4; column-fill: auto }

article { width: 60em; height: auto; columns: 4; column-fill: balance }
p { break-after: column }

article { width: 60em; height: auto; columns: 4; column-fill: balance }

8 Overflow

8.1 Overflow inside multicol elements

Floated or in-flow content that extends into column gaps or neighboring columns — e.g., long words or images — is not clipped and may therefore cause overflow.

8.2 Pagination and overflow outside multicol elements

Content and column rules that extend outside column boxes at the edges of the multi-column element are clipped according to the overflow property.

A multicol element can have more columns than it has room for due to:

• a declaration that constrains the column height (e.g., using height or max-height). In this case, additional column boxes are created in the inline direction
• the size of the page. In this case, additional column boxes are moved to the next page(s).
• explicit column breaks. In this case, additional column boxes are created in the inline direction for continuous media, and additional column boxes are moved to the next page(s) for paged media.

Columns that appear outside the multicol element in continuous media are called overflow columns.

div {
  max-height: 5em;
  overflow: visible;
}

p {
  break-after: column;
}

Acknowledgments

This document is based on several older proposals and comments on older proposals. Contributors include:

Alex Mogilevsky, Andy Clarke, Anton Prowse, Bert Bos, Björn Höhrmann, Cédric Savarese, Chris Lilley, Chris Wilson, Daniel Glazman and Dave Raggett, David Hyatt, David Singer, David Woolley, Elika Etemad, Giovanni Campagna, Ian Hickson. Joost de Valk, Kevin Lawver, L. David Baron, Markus Mielke, Melinda Grant, Michael Day, Øyvind Stenhaug, Peter Linss, Peter-Paul Koch, Robert O’Callahan, Robert Stevahn, Sergey Genkin, Shelby Moore, Steve Zilles, Sylvain Galineau, Tantek Çelik, Till Halbach,

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