CSS Multi-column Layout Module

Abstract

This specification describes multi-column layouts in CSS, a style sheet language for the web. Using functionality described in the specification, content can be flowed into multiple columns with a gap and a rule between them.

Status of this document

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

This document was produced by the CSS Working Group as a Candidate Recommendation.

A Candidate Recommendation is a document that has been widely reviewed and ready for implementation. W3C encourages everybody to implement this specification and return comments to the (archived) public mailing list www-style@w3.org (see instructions). When sending e-mail, please put the text “css3-multicol” in the subject, preferably like this: “[css3-multicol] …summary of comment…”

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

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

This document has been a Working Draft in the CSS Working Group for several years. Multi-column layouts are traditionally used in print. On screen, multi-column layouts have been considered experimental, and implementation and use experience was deemed necessary in order to proceed. Several implementations have occurred over the past years, and this draft incorporates useful feedback from implementors as well as authors and users.

This document will remain Candidate Recommendation at least until 17 June 2010. See the CR exit criteria.

Table of contents

• 1. Introduction
• 2. The multi-column model
• 3. The number and width of columns
  • 3.1. ‘column-width’
  • 3.2. ‘column-count’
  • 3.3. ‘columns’
  • 3.4. Pseudo-algorithm
  • 3.5. Stacking context
• 4. Column gaps and rules
  • 4.1. ‘column-gap’
  • 4.2. ‘column-rule-color’
  • 4.3. ‘column-rule-style’
  • 4.4. ‘column-rule-width’
  • 4.5. ‘column-rule’
• 5. Column breaks
• 6. Spanning columns
  • 6.1. ‘column-span’
• 7. Filling columns
• 8. Overflow and multicol elements
  • 8.1. Overflow inside multicol elements
  • 8.2. Overflow outside multicol elements
• 9. Conformance
  • 9.1. CR exit criteria
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index

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 multi-column element 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 the appear; column rows 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, then 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.

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.

<html>
<style type="text/css">
div {
  column-width: 15em;
  column-gap: 2em;   /* shown in yellow */            
  column-rule: 4px solid green;
  padding: 5px;      /* shown in blue */
}
p { margin: 0; padding: 0 }
img { display: none }
</style>
<body>
<div>
<p>Ab cde fgh i jkl. Mno 
pqr stu vw xyz. A bc
<img src=...>
def g hij
...
</div>
</body>
</html>

Column boxes act as the containing block for their content. 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 containing blocks 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.

3. The number and width of columns

Finding the number and width of columns is fundamental when laying out multi-column content. When the block direction is unconstrained and no column breaks are added through style sheets, these two properties determine the outcome:

• ‘column-count’
• ‘column-width’

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

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). 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.

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, all length values (in horizontal text these are: ‘width’, ‘column-width’, ‘column-gap’, and ‘column-rule-width’) must 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). There are two other variables in the pseudo-algorithm:

• available-width: if the used width of the multi-column element has not been determined when the ‘column-count’ and ‘column-width’ must be determined (e.g., if it is floating with a ‘width’ of ‘auto’ as per CSS 2.1 section 10.3.5) this variable is unknown, otherwise it is the same as the used width of the multi-column element. In vertical text, the used height replaces used width in this calculation.
• shrink-to-fit: this variable represents the result of a shrink-to-fit computation. CSS does not define the exact algorithm.

Two assumptions are being made by the pseudo-algorithm:

• that the block direction is unconstrained
• that no column breaks are added through style sheets

The floor() function rounds a number to the next smaller integer.

(01)  if ((column-width = auto) and (column-count = auto)) or
(02)     ((available-width = unknown) and (column-count = auto)) then
(03)    exit; /* no columns */
(04)  fi
(05)  
(06)  if (available-width = unknown) and (column-count != auto) and (column-width != auto) then
(07)    N := column-count;
(08)    W := column-width;
(09)    exit;
(10)  fi
(11)  
(12)  if (available-width = unknown) then
(13)    available-width := shrink-to-fit;
(14)  fi
(15)  
(16)  if (column-width = auto) then
(17)    if ((column-count - 1) * column-gap < available-width) then
(18)      N := column-count;
(19)      W := (available-width - ((N - 1) * column-gap))/N;
(20)    elsif (column-gap >= available-width) then
(21)      N := 1;
(22)      W := available-width;
(23)    else
(24)      N := floor(available-width/column-gap);
(25)      W := (available-width - ((N - 1) * column-gap))/N;
(26)    fi
(27)  elsif (column-count = auto) then
(28)    if (column-width >= available-width) then
(29)      N := 1
(30)      W := available-width;
(31)    else
(32)      N := floor((available-width + column-gap) / (column-width + column-gap));
(33)      W := ((available-width + column-gap) / N) - column-gap;
(34)    fi
(35)  elsif (column-width >= available-width) then
(36)    N := 1
(37)    W := available-width;
(38)  else
(39)    N := floor((available-width + column-gap) / (column-width + column-gap));
(40)    W := ((available-width + column-gap) / N) - column-gap;
(41)  fi

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

Note that, in most cases, only one of ‘column-width’ and ‘column-count’ affect the layout. If ‘column-width’ has a value other than ‘auto’, ‘column-count’ will be ignored. The only case when both ‘column-width’ and ‘column-count’ can affect the layout is for element where the width of the element has not been determined. This can, e.g., be the case for table cells and floats.

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.

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 length of the columns. 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 column rule will overlap adjacent column boxes. Column rules are painted just above the background of the multicol element. This allows child elements with ‘z-index’ values to be on top of column rules. Column rules are only drawn between two columns that both have content.

4.1. ‘column-gap’

The ‘column-gap’ property sets the gap between columns. If there is a column rule between columns, it will appear in the middle of the gap.

The ‘normal’ value is UA-specific. A value of ‘1em’ is suggested.

Column gaps cannot be negative.

4.2. ‘column-rule-color’

This property sets the color of the column rule. The <color> values are defined in [CSS21].

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 defined in [CSS21].

The ‘inset’ keyword value is shown like the ‘ridge’ value. The ‘outset’ value is shown like ‘groove’.

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.

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.

These properties describe page/column break behavior before/after/inside the generated box. These 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.

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:

1

The element does not span multiple columns.

all

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.

An element that spans more than one column is called a spanning element.

h2 { column-span: all; background: silver } 
img { display: none }

7. Filling columns

There are two strategies for filling columns: columns can either be balanced, or not. If columns are balanced, UAs should minimize the variation in column length. Otherwise, columns are filled sequentially and will therefore end up having different lengths. In any case, the user agent should try to honor the ‘widows’ and ‘orphans’ properties.

The values are:

balance

Balance content equally between columns, if possible.

auto

Fills columns sequentially.

In continuous media, this property will only be consulted if the length of columns has been constrained. Otherwise, columns will automatically be balanced.

8. Overflow and multicol elements

8.1. Overflow inside multicol elements

Content in the normal flow that extends into column gaps (e.g., long words or images) is clipped in the middle of the column gap.

8.2. Overflow outside multicol elements

Content that extend outside column boxes at the edges of the multi-column element is clipped according to the ‘overflow’ property.

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

• Constrained column height. A declaration that constrains the column height (e.g., using ‘height’) must be honored, if possible. In paged media, the column height is constrained by the size of the page.
• Explicit column breaks. Explicit column breaks may create more columns than there is room for inside the multicol element.

In continuous media, columns that do not fit within the multicol element are added in the inline direction.

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

p { 
  break-after: column;
}

In paged media, columns that do not fit within the page are moved to the next page.

p { 
  break-after: column;
}

9. Conformance

Conforming UAs must flow the content of an element into several columns according to this specification.

The 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]

9.1. CR exit criteria

As described in the W3C process document, a Candidate Recommendation (CR) is a specification that W3C recommends for use on the Web. The next stage is Recommendation, when the specification is sufficiently implemented.

For this specification to be proposed as a W3C Recommendation, the following conditions shall be met. There must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

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

interoperable

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

implementation

a user agent which:

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

A minimum of sixth months of the CR period must have elapsed. This is to ensure that enough time is given for any remaining major errors to be caught.

Features will be dropped if two or more interoperable implementations are not found by the end of the CR period.

Features may/will also be dropped if adequate/sufficient (by judgment of CSS WG) tests have not been produced for those feature(s) by the end of the CR period.

Acknowledgments

This document is based on several older proposals and comments on older proposals. Contributors include Sylvain Galineau, Giovanni Campagna, David Hyatt, David Singer, David Woolley, Elika Etemad, Björn Höhrmann, Joost de Valk, Peter-Paul Koch, Till Halbach, Cédric Savarese, Andy Clarke, Robert O'Callahan, Markus Mielke, Alex Mogilevsky, Sergey Genkin, Michael Day, Melinda Grant, Kevin Lawver, David Baron, Bert Bos, Dave Raggett, Chris Wilson, Robert Stevahn, Peter Linss, Chris Lilley, Steve Zilles, Tantek Çelik, Daniel Glazman and Ian Hickson.

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 8 September 2009. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2009/CR-CSS2-20090908

[RFC2119]

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

Other references

Index

• break-after, 5.
• break-before, 5.
• break-inside, 5.
• column box, 2.
• column-count, 3.2.
• column-fill, 7.
• column-gap, 4.1.
• column-rule, 4.5.
• column-rule-color, 4.2.
• column-rule-style, 4.3.
• column-rule-width, 4.4.
• columns, 3.3.
• column-span, 6.1.
• column-width, 3.1.
• multi-column element, 2.
• spanning element, 6.1.

Property index