CSS3 module: Multi-column layout

Abstract

This module describes multi-column layout in CSS. It builds on the CSS3 Box model module and adds functionality to flow the content of an element into multiple columns.

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/.

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

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

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

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

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.

Table of contents

• 1. Dependencies on other modules
• 2. Introduction
• 3. The multi-column model
• 4. The number and width of columns
  • 4.1. ‘column-width’
  • 4.2. ‘column-count’
  • 4.3. ‘columns’
  • 4.4. Pseudo-algorithm
  • 4.5. Stacking context
• 5. Column gaps and rules
  • 5.1. ‘column-gap’
  • 5.2. ‘column-rule-color’
  • 5.3. ‘column-rule-style’
  • 5.4. ‘column-rule-width’
  • 5.5. ‘column-rule’
• 6. Column breaks
  • 6.1. ‘column-break-before’
  • 6.2. ‘column-break-after’
• 7. Spanning columns
  • 7.1. ‘column-span’
• 8. Filling columns
• 9. Overflow and multi-column elements
• 10. Conformance
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index

1. Dependencies on other modules

This CSS3 module depends on the following other CSS3 modules:

• CSS3 Box Model [CSS3BOX]
• CSS3 Backgrounds and Borders Module [[!CSS3BACKGROUND]]
• CSS3 Color Module [CSS3COLOR]
• CSS3 Paged Media [CSS3PAGE]
• CSS3 Generated Content for Paged Media [CSS3GCPM]

It has non-normative references to the following other CSS3 modules:

• [[!CSS3VALUES]]

2. 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. By removing presentational table markup from documents they can 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 both properties in one declaration. The rule in the example below sets the same property/value pairs as the rules in the two examples above:

body { columns: 2 12em }

Another group of properties introduced in this module describe gaps and rules between columns. Here is a simple example:

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

The first declaration in the example above sets the gap between two adjacent columns to be 1em. Column gaps are similar to padding areas. In the middle of the gap there will be a rule which is described by the ‘column-rule’ property. The values of the ‘column-rule’ property are similar to those of the CSS border properties. Like ‘border’, ‘column-rule’ is also a compound property. The example above is therefore identical to this:

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

A third group of properties indicate where column breaks should occur:

h1 { column-break-before: always }
h2 { column-break-after: avoid }

Finally, ‘column-fill’ and ‘column-span’ give style sheets a wider range of visual expressions in multi-column layouts:

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

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

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.

3. The multi-column model

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. The content of an element with more than one column is flowed into column boxes in the block direction, and column boxes (or columns, for short) are flowed into the content box of the multi-column element in the inline-progression-direction.

An element that has, or possibly has, more than one column is called a multi-column element (or simply multicol). That is, multi-column elements have ‘column-count’ or ‘column-width’ different from ‘auto’. All columns of a multi-column element have the same column width, column height, and the same gap between them. A multi-column element establishes a new block formatting context, as per CSS 2.1 section 9.4.1.

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.

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.

Between adjacent columns of the same element, there can be column gaps and column rules.

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

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

img { display: block; float: right }

Column boxes act like block-level, table cell, and inline-block boxes as per CSS 2.1, section 10.1, item 2 [CSS21]. As a result, floats are positioned with regard to the column box. However, the column box does not establish a contaning block for elements with ‘position: fixed’ or ‘position: absolute’.

img { display: block; width: 100% }

The top margin of the first element and the bottom margin of the last element will collapse with the margins of the multi-column element as per the normal rules for collapsing.

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

4. 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’.

4.1. ‘column-width’

The ‘column-width’ property 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).

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.

4.2. ‘column-count’

The ‘column-count’ property describes the ideal number of columns into which the content of the element will be flowed.

An integer value sets the ideal number of columns into which the content of the element will be flowed.

body { column-count: 3 }

4.3. ‘columns’

This is a shorthand property for setting ‘column-width’ and ‘column-count’ at the same place in the style sheet. If two legal values (one for ‘column-count’ and one for ‘column-width’) are specified, both properties are set to their respective specified value. If only one value is specified, the following applies:

• A length value or ‘auto’ sets ‘column-width’ to the specified valued and ‘column-count’ to its initial value.
• An integer value sets ‘column-count’ to the specified value and ‘column-width’ to its initial value.

4.4. 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 content 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 varible is unknown, otherwise it is the same as the content width of the multi-column element.
• 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.

if ((column-width = auto) and (column-count = auto)) or
   ((available-width = unknown) and (column-count = auto)) then
  exit; /* no columns */
fi

if (available-width = unknown) and (column-count != auto) and (column-width != auto) then
  N := column-count;
  W := column-width;
  exit;
fi

if (available-width = unknown) then
  available-width := shrink-to-fit;
fi

if (column-width = auto) then
  if ((column-count - 1) * column-gap < available-width) then
    N := column-count;
    W := (available-width - ((N - 1) * column-gap))/N;
  elsif (column-gap >= available-width) then
    N := 1;
    W := available-width;
  else
    N := floor(available-width/column-gap);
    W := (available-width - ((N - 1) * column-gap))/N;
  fi
elsif (column-count = auto) then
  if (column-width >= available-width) then
    N := 1
    W := column-width;
  else
    N := floor((available-width + column-gap) / (column-width + column-gap));
    W := ((available-width + column-gap) / N) - column-gap;
  fi
elsif ((column-count * column-width) + ((column-count - 1 ) * column-gap) <= available-width) then
  N := column-count;
  W := column-width;
elsif (column-width >= available-width) then
  N := 1
  W := column-width;
else
  N := floor((available-width + column-gap) / (column-width + column-gap));
  W := ((available-width + column-gap) / N) - column-gap;
fi

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.

4.5. Stacking context

All column boxes in a multi-column element are in same stacking context. If content from two columns overlaps while in same z-order they are drawn in tree order. No new behavior is introduced over CSS2.1 Appendix E.

5. Column gaps and rules

Column gaps and rules are placed between columns of the same multi-column 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 multi-column element). A column rule is placed in the middle of the column gap. 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 only drawn between columns that have content in the normal flow.

5.1. ‘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.

5.2. ‘column-rule-color’

This property sets the color of the column rule. The <color> values are defined in [CSS3COLOR].

Conforming user agents are only required to support the subset of color values defined in [CSS21].

5.3. ‘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’.

Note that the ‘none’ value forces the computed value of ‘column-rule-width’ to be ‘0’.

5.4. ‘column-rule-width’

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

5.5. ‘column-rule’

This property is a shorthand property for setting ‘column-rule-width’, ‘column-rule-style’, and ‘column-rule-color’ at the same place in the style sheet.

6. 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. CSS3 Paged Media [CSS3PAGE] describes rules for breaking content into pages. This specification prescribes the same rules for column breaks, with the following differences:

• The properties for indicating column breaks are ‘column-break-before’ and ‘column-break-after’ (instead of ‘page-break-before’ and ‘page-break-after’). The ‘page-break-inside’ property can be used to indicate that a column break should be avoided inside the element.
• A page break will always cause a column break; the opposite is not the case. Consequently, when both a page break and a column break is specified, a page break will occur.

Because the rules for column breaks are similar to rules for page breaks, margins will be set to zero where column breaks occur.

6.1. ‘column-break-before’

6.2. ‘column-break-after’

7. Spanning columns

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

7.1. ‘column-span’

This property describes how many columns an element spans across.

1

The element does not span multiple columns.

all

The element spans across all columns. Content in the normal flow that appear 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 }

8. Filling columns

There are two strategies for filling columns: columns can either be balanced, or not. If columns are balanced, UAs will try to balance content between different columns. Otherwise, columns are filled sequentially. 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 continous media, this property will only be consulted if the length of columns has been constrained. Otherwise, columns will will automatically be balanced.

9. Overflow and multi-column elements

Content that extend into column gaps (e.g., long words and floats that are higher and/or wider than the column box) is clipped in the middle of the column gap. Content that extend outside column boxes at the edges of the multi-column element is clipped according to the ‘overflow’ property.

However, some content (e.g., floats) may intrude into other columns.

A multi-column element can have more columns than it has room for due to:

• A constrained column height. The height can be constrained by a rule on the multi-column element, or, in paged media, by the size of the page.

  In this example, the height of the multi-column element has been constrained to a maximum height. Also, the style sheet specifies that overflowing content should be visible:

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

  As a result, the number of columns is increased.

  Ab cde fgh i jkl. Mno
  pqr stu vw xyz. A bc
  def g hij klm nopqrs
  tuv wxy z. Abc de fg
  hi jklmno. Pqrstu vw
  

  x yz. Abc def ghi jkl.
  M nop qrst uv wx yz.
  Ab cde fgh i jkl. Mno
  pqr stu vw xyz. A bc
  def g hij klm nopqrs

  tuv wxy z. Abc de fg
  hi jklmno. Pqrstu vw
  x yz. Abc def ghi jkl.
  M nop qrst uv wx yz.
  Ab cde fgh i jkl. Mno
  

  pqr stu vw xyz. A bc
  def g hij klm nopqrs
  tuv wxy z. Abc de fg
  hi jklmno. Pqrstu vw
  x yz. Abc def ghi jkl.
  
• Explicit column breaks.

  In this example, explicit column breaks are generated after paragraphs:

  p { 
    column-break-after: always;
  }
  

  As a result, the number of columns is increased:

  Ab cde fgh i jkl. Mno
  pqr stu vw xyz.

  Ab cde fgh i jkl. Mno
  pqr stu vw xyz. A bc
  def g hij klm nopqrs
  tuv wxy z.

  Ab cde fgh i jkl mno.
  

  Pqr stu vw xyz. A bc
  def g hij klm nopqrs
  tuv wxy z. Abc de fg
  hi jklmno pqr stu v
  wx yz.

In paged media, columns for which there is not room within the page are are moved to the next page.

p { 
  column-break-after: always;
}

10. Conformance

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

User agents that support CSS3 Backgrounds and Borders [[!CSS3BACKGROUND]] must support ‘column-gap-image’; for other UAs this property is optional.

For the ‘column-rule-color’ property, conforming user agents are only required to support the subset of color values defined in [CSS21].

Acknowledgments

This document is based on several older proposals and comments on older proposals. Contributors include 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. 11 April 2006. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2006/WD-CSS21-20060411

[CSS3BOX]

Bert Bos. CSS3 module: The box model. 24 October 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/WD-css3-box-20021024

[CSS3COLOR]

Tantek Çelik; Chris Lilley. CSS3 Color Module. 14 May 2003. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2003/CR-css3-color-20030514

[CSS3GCPM]

Håkon Wium Lie. CSS Generated Content for Paged Media Module. 19 September 2006. W3C Working Draft. (Work in progress) URL: http://www.w3.org/TR/2006/WD-css3-gcpm-20060919

[CSS3PAGE]

Håkon Wium Lie; Melinda Grant. CSS3 Paged Media Module. 25 February 2004. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2004/CR-css3-page-20040225

Other references

Index

• column box, #
• column-break-after, #
• column-break-before, #
• column-count, #
• column-fill, #
• column-gap, #
• column gaps, #
• column height, #
• column-rule, #
• column-rule-color, #
• column rules, #
• column-rule-style, #
• column-rule-width, #
• columns, #
• column-span, #
• column-width, #
• column width, #
• multicol, #
• multi-column element, #
• spanning element, #

Property index