Flexible Box Layout Module

Abstract

The draft describes a CSS box model optimized for interface design. It provides an additional layout system alongside the ones already in CSS. [CSS21] In this new box model, the children of a box are laid out either horizontally or vertically, and unused space can be assigned to a particular child or distributed among the children by assignment of “flex” to the children that should expand. Nesting of these boxes (horizontal inside vertical, or vertical inside horizontal) can be used to build layouts in two dimensions. This model is based on the box model in the XUL user-interface language used for the user interface of many Mozilla-based applications (such as Firefox).

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-flexbox” in the subject, preferably like this: “[css3-flexbox] …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 is the first public working draft in the “css3-flexbox” series.

Table of contents

• 1 Overview
• 2 Orientation
• 3 Display order
• 4 Alignment
• 5 Flexibility
• 6 Packing along the box axis
• 7 Multiple Lines
  • 7.1 Multiple Lines and alignment
• 8 Relationship to Inlines and blocks
• Acknowledgments
• References
  • Normative references
• Index
• Property index

1 Overview

Flexible boxes lay out their children using a constraint-based system that supports both relative flexible sizing and intrinsic sizing. Elements within a box may be intrinsically sized yet have their size increased if additional space is available in the container or have their size reduced if less space is available. In addition, the position and order of elements within a box may be modified.

In CSS, flexible boxes (often referred to only as boxes in this specification) may be created by setting the ‘display’ property. A block-level box can be specified with a value of ‘box’ and an inline box can be specified using a value of inline-box.

A block-level box placed inside a block level element is positioned and sized as if it were any other block. An inline-level box placed inside another element is positioned and sized like an inline-block.

Inline children of a flexible box are wrapped in anonymous blocks. All other children are block-level. Elements within boxes are sized intrinsically. This means that the size is determined by the size needed for its content, or, in the case of replaced elements, the size of the replaced element. The intrinsic size of non-replaced elements may be affected by widths and heights specified on the descendants. When determining the intrinsic size of an element, the calculation is done without adding any line breaks, except those necessary (e.g. for content with a white-space of ‘pre’).

Children of boxes may specify their width and height using the ‘width’ and ‘height’ properties. The width and height of replaced elements is computed as with replaced elements in inline contexts. Otherwise, if the width is set to ‘auto’, then the used value is the intrinsic size, which may be calculated using a shrink-to-fit algorithm, as with inline-block elements, i.e., it will be given the minimum amount of space required to lay out the element without unnecessary line breaks. Similarly, if the height is set to ‘auto’, then the used value in a similar manner.

An unnested box placed inside an enclosing block will obey all the usual sizing rules of the block. For example, setting the width of an unnested box to 100% ensures that the box is the maximum width permitted by the enclosing block (ignoring any margins, border and padding on that box).

The difference between boxes and blocks is in how the children of the element are laid out. Children of boxes are all block-level (inline-level contents get wrapped in anonymous blocks, as described in the section on inlines and blocks). These blocks then get stacked in the direction specified by the ‘box-orient’ and ‘box-direction’ properties, and order specified by the ‘box-ordinal-group’ property. They are aligned within the box according to the ‘box-align’ property. The boxes may then flex or be distributed as specified by the ‘box-flex’, ‘box-flex-group’, ‘box-pack’ and ‘box-lines’ properties.

The ‘float’ and ‘clear’ properties do not apply to children of box elements, so the used value for these properties is ‘none’. These properties do apply to the box itself if it is not a child of another box.

   <p style="width: 400px;">
     <button style="width: 200px;">Child 1</button>
     <button style="width: 100px;">Child 2</button>
   </p>

Elements within a box may use the ‘overflow’ property to control whether a scrolling mechanism appears when the children of a box overflow. A scrolling mechanism may be displayed when flexible elements are reduced below their minimum intrinsic size when the overflow property is set to auto or scroll. If overflow is hidden, the element will be clipped instead. Note that the initial value is visible, which is typically not the preferred effect in user interfaces.

2 Orientation

A box may lay out its children either horizontally or vertically.

horizontal

The box displays its children from left to right in a horizontal line.

vertical

The box displays its children from stacked from top to bottom vertically.

inline-axis

The box displays its children along the inline axis.

block-axis

The box displays its children along the block axis.

The inline and block axes are the writing-mode dependent keywords which, in English, map to horizontal and vertical respectively.

3 Display order

Children within a horizontally oriented box are, by default, displayed from left to right in the same order as they appear in the source document. Children within a vertically oriented box are displayed top to bottom in the same order. The box-direction and box-ordinal-group properties may be used to change this ordering.

The ‘box-direction’ property specifies the direction in which children of a box are displayed.

normal

A box with a computed value of horizontal for box-orient displays its children from left to right. A box with a computed value of vertical displays its children from top to bottom.

reverse

A box with a computed value of horizontal for box-orient displays its children from right to left. A box with a computed value of vertical displays its children from bottom to top.

If the computed value for the box's ‘direction’ property is ‘normal’, a child's left edge is placed next to the right edge of the previous child, or the left edge of the box for the first child. If the computed value for the box's ‘direction’ property is ‘reverse’, a child's right edge is placed next to the left edge of the previous child, or the right edge of the box for the first child.

When the computed value for the overflow property is ‘visible’, ‘scroll’ or ‘auto’, the content may overflow the container. If the computed value for direction is normal, the content will overflow over the right or bottom side. If the computed value for direction is reverse, the content will overflow over the left or top side.

The children of a box element may be assigned to ordinal groups using the ‘box-ordinal-group’ property. This property is a natural number value with an initial value is 1.

Ordinal groups can be used in conjunction with the ‘box-direction’ property to control the order in which the direct children of a box appear. When the computed box-direction is normal, a box will display its elements starting from the lowest numbered ordinal group and ensure that those elements appear to the left (for horizontal boxes) or at the top (for vertical boxes) of the container. Elements with the same ordinal group are flowed in the order they appear in the source document tree. In the reverse direction, the ordinal groups are examined in the same order, except the elements appear reversed.

    #div1 { display: box; }
    #span1 { box-ordinal-group: 2; }
    #span3 { box-ordinal-group: 2; }
    #span4 { box-ordinal-group: 1; }

    <div id="div1">
      <span id="span1" >Sentence One</span>
      <span id="span2" >Sentence Two</span>
      <span id="span3" >Sentence Three</span>
      <span id="span4" >Sentence Four</span>
    </div>

    span2 span4 span1 span3

Elements within a box can use the ‘visibility’ property to render themselves invisible. Boxes behave like tables in that the value collapsed can be used to specify that an element within a box should not take up any space at all. The computed width and height of a collapsed element are both 0, and the element is not considered when calculating flexibility. Other non-collapsed flexible elements may expand as needed to fill in any space left open by a collapsed element.

4 Alignment

When the size of the containing box is larger than the size of a child, extra space will be available. The ‘box-align’ property specifies how a box's children are placed and aligned along the direction perpendicular to the box orientation, and where the extra space, if any, is positioned. For horizontal orientation, it specifies how the children are positioned vertically. For vertical orientation, it specifies how the children are positioned horizontally.

The amount of extra space may be different for each child. For example, if the containing box has a height of 200 pixels, and it contains two children at 100 and 150 pixels respectively, there will be 100 pixels of extra space for the first child and 50 pixels of space for the second child.

The following values are valid for the box-align property, but see the text afterward for more specifics as to how children are positioned.

start

For normal direction boxes, the top edge of each child is placed along the top of the box. Extra space is placed below the element. For reverse direction boxes, the bottom edge of each child is placed along the bottom of the box. Extra space is placed above the element.

end

For normal direction boxes, the bottom edge of each child is placed along the bottom of the box. Extra space is placed above the element. For reverse direction boxes, the top edge of each child is placed along the top of the box. Extra space is placed below the element.

center

Any extra space is divided evenly, with half placed above the child and the other half placed after the child.

baseline

If this box orientation is inline-axis or horizontal, all children are placed with their baselines aligned, and extra space placed before or after as necessary. For block flows, the baseline of the first non-empty line box located within the element is used. For tables, the baseline of the first cell is used. The children, once aligned on their baselines, should then be placed into the box so that the child with the earliest extent margin has its top margin edge (or bottom margin edge for reverse direction boxes) flush with the top (or bottom) edge of the box's content area. If the box does not have an ‘auto’ height, overflow will always be on the bottom (or top for reverse direction boxes) edge. If the box orientation is block-axis or vertical, then baseline is interpreted as center.

stretch

The height of each child is adjusted to that of the containing block. However, note the text below.

The following rules should be applied when determining the size of children inside boxes in the dimension opposite to the orientation of the box. Exceptions to these rules are noted under each of the possible values of ‘box-align’ below.

All normal flow children of boxes will attempt to satisfy the following sizing equation when placed inside vertical boxes if and only if at least one of the specified values is auto:

‘margin-left’ + ‘border-left-width’ + ‘padding-left’ + ‘width’ + ‘padding-right’ + ‘border-right-width’ + ‘margin-right’ = width of containing box

and a similar equation for height when placed inside horizontal boxes:

‘margin-top’ + ‘border-top-width’ + ‘padding-top’ + ‘height’ + ‘padding-bottom’ + ‘border-bottom-width’ + ‘margin-bottom’ = height of containing box

If all the values in the equation are specified (i.e., if there are no auto values), then the equation above does not apply, and the specified values should be used. If applying the equation results in a negative value for width/height, then that value is set to 0, and those values should be used.

If the value of ‘box-align’ is stretch, then elements obey the sizing rules specified above. This is the default value. If both width/height and one or both margin values are ‘auto’, the ‘auto’ margins are set to 0 and the equation is used to find a value for width/height. Otherwise, if both margins are ‘auto’, then the equation is solved under the additional constraint that the two margins must get the same value. If neither the width/height or either margin is auto, then the extra space is positioned as if box-align was start.

If the value of ‘box-align’ is start, center, baseline, or end, and a value of ‘auto’ is specified for width/height, then the intrinsic size (or shrink to fit size) of the element should be used. If one margin has the value of auto, then the equations above should be used. If both margins have a value of auto then the above equations should be used, with the space being divided evenly between the two margins. These rules are similar to the rules for replaced elements inside blocks.

If the above would cause the width or height to greater than the computed maximum width or height, then the width or height is set to the maximum width or height, and the auto margins increased by the extra amount left over. If both margins are auto, they should both be increased by the same amount. If neither margin is auto, the extra space is placed as necessary depending on the box-align value.

Once the sizes are computed, the margin boxes of the elements should be aligned at the start, center, baseline, or end of the box. For horizontal boxes, this typically means the top, center, baseline or bottom of the box. For vertical boxes, the start and end alignments depend upon the direction. For LTR, the elements would be aligned at the left side of the box for start and the right side for end. For RTL, the elements would be aligned at the right of the box for start and the left side for end.

5 Flexibility

Each element directly within a box may be made either flexible and inflexible. Flexible elements may grow when the containing box has extra space available after the size of all of its children have been computed, and shrink if the size would cause the containing box to overflow, yet the preferred width of the flexible element is larger than its minimum width. Inflexible elements do not change in size, even when there is extra space left over in the box.

The flexibility only affects the size in the axis along the box's orientation. (e.g. the flexibility only affects the width in a horizontal box, and only the height in a vertical box.)

An element is flexible when the box-flex property is specified. The box-flex property is a floating point value representing the flexibility of the element. Its initial value is 0, which indicates that the element is inflexible. Elements that are flexible can shrink or grow as the box shrinks and grows. Whenever there is extra space left over in a box, the flexible elements are expanded to fill that space. All flex is relative. For example, a child with a box-flex of 2 is twice as flexible as a child with a box-flex of 1. A negative value for box-flex is not allowed.

Flexible elements can be assigned to flex groups using the ‘box-flex-group’ property. This property is a natural number value (the first flex group is 1 and higher values are later flex groups). The initial value is 1.

In a horizontally oriented box, the preferred width of each child is computed. If the width of the margin box of each child is equal to the width of the containing block, then there is no extra space available, so the preferred widths are used for each child. If the width of the margin box adds up to a value smaller than the width of the containing block, then extra space is available. This extra space is divided up among the flexible children, as described below. If the width of the margin box adds up to a value larger than the width of the containing block, then the flexible children shrink as much as necessary to prevent overflow.

Flexibility only applies to elements in normal flow. As absolute and fixed positioned elements are not in flow, any flexibility or flexgroup specified on them is ignored. In addition, as the ‘float’ property does not apply to children of flexible boxes, they are considered part of normal flow and flexibility does apply.

When dividing up extra space, first take all elements within the first flex group. Each element within that group should be given extra width based on the ratio of that element's flexibility compared to the flexibility of other elements within the same flex group.

However, if the preferred width of the element plus the extra width allotted to it is larger than the maximum width of the element, then the width is set to that maximum width, and any remaining extra width beyond that is divided up among the other children.

    #div1 { display: box; width: 300px; }
    #button1 { box-flex: 1.0; width: 100px; }
    #button2 { box-flex: 2.0; width: 140px; }

    <div id="div1">
      <button id="button1">Hello</button>
      <button id="button2">Goodbye</button>
    </div>

More specifically, the percentage of extra space that an element may receive is calculated as follows:

‘box-flex of child’ / ‘total of box-flex values of child and all siblings’

If the width of all flexible children within the group has been increased to their maximum widths, the process repeats for the children within the next flex group, using any space left over from the previous flex group. Once there are no more flex groups, and there is still space remaining, the extra space is divided within the containing box according to the box-pack property.

If the box would overflow after the preferred width of the children have been computed, then width is removed from flexible elements in a manner similar to that used when adding extra space. Each flex group is examined in turn and space is removed according to the ratio of the flexibility of each element. Elements do not shrink below their minimum widths.

If all children have been shrunk to their minimum sizes, then the box overflows, although if the box-lines property is set to multiple, the box may be able to move elements to additional lines to prevent this.

For vertically oriented boxes, the algorithm as described above is similar except using the height instead of the width.

When a child box of a horizontally oriented box contains an inline element, it is likely that shrinking the width of the element due to flexibility may cause the inline element to grow in height, as the text within it may need to wrap to additional lines.

    #div1 { display: box; box-orient: vertical; height: 200px }

    <div id="div1">
      <button>Cat</button>
      <button style="box-flex: 1">Piranha</button>
      <button>Antidisestablishmentarianism</button>
    </div>

    <div style="display: box; box-orient: vertical;">
      <button style="box-flex: 1; height: 1000px;">
        Cat
      </button>
    </div>

    #div1 { display: box; }
    #iframe1 { box-flex: 1; min-width: 100px; max-width: 300px; height: 300px; }

    <div id="div1">
      <iframe id="iframe1" src="http://www.mozilla.org/"/>
    </div>

    <p style="display: box;">
      <button style="box-flex: 1; max-width: 50px;">Child 1</button>
      <button style="box-flex: 1; min-width: 50px;">Child 2</button>
    </p>

6 Packing along the box axis

When all of the elements within a box are inflexible or when all elements have grown to their maximum sizes and can stretch no further, extra space may be left over in the box. The box-pack property may be used to dictate how any additional space along the box-axis should be distributed between elements. The box-pack property does not affect the position of elements in the opposite direction. That is, box-pack affects only the horizontal position in horizontally oriented boxes and only the vertical position in vertically oriented boxes.

start

For normal direction boxes, the left edge of the first child is placed at the left side, with all extra space placed after the last child. For reverse direction boxes, the right edge of the last child is placed at the right side, with all extra space placed before the first child.

end

For normal direction boxes, the right edge of the last child is placed at the right side, with all extra space placed before the first child. For reverse direction boxes, the left edge of the first child is placed at the left side, with all extra space placed after the last child.

center

The extra space is divided evenly, with half placed before the first child and the other half placed after the last child.

justify

The space is divided evenly in-between each child, with none of the extra space placed before the first child or after the last child. If there is only one child, treat the pack value as if it were start.

    <p style="box-align: center; box-pack: center; width: 300px; height: 300px;">
      <button>centered</button>
    </p>

7 Multiple Lines

By default a horizontal box will lay out its children in a single row, and a vertical box will lay out its children in a single column. This behavior can be changed using the box-lines property. The default value is single, which means that all elements will be placed in a single row or column, and any elements that don't fit will simply be considered overflow.

If a value of multiple is specified, however, then the box is allowed to expand to multiple lines (that is, multiple rows or columns) in order to accommodate all of its children. The box must attempt to fit its children on as few lines as possible by shrinking all elements down to their minimum widths or heights if necessary.

If the children in a horizontal box still do not fit on a line after being reduced to their minimum widths, then children are moved one by one onto a new line, until the elements remaining on the previous line fit. This process can repeat to an arbitrary number of lines. If a line contains only a single element that doesn't fit, then the element should stay on that line and overflow out of the box. The later lines are placed below the earlier lines in normal direction boxes and above in reverse direction boxes. The height of a line is the height of the largest child in that line. No additional space appears between the lines apart from the margins on the largest elements in each line. For calculating the height of a line, margins with a computed value of auto should be treated as having a value of 0.

A similar process occurs for children in a vertical box. Later lines in normal direction boxes are placed to the right of earlier lines and to the left in reverse direction boxes.

Once the number of lines has been determined, elements with a computed value for box-flex other than 0 stretch as necessary in an attempt to fill the remaining space on the lines. Each line computes flexes independently, so only elements on that line are considered when evaluating flex and flex-groups. The packing of elements in a line, as specified by the box-pack property, is also computed independently for each line.

    #div1 {
      display: box;
      box-lines: multiple;
      width: 300px;
    }
    button {
      box-flex: 1.0;
      width: 100px;
    }

    <div id="div1">
      <button id="button1">Elephant</button>
      <button id="button2">Tiger</button>
      <button id="button1">Antelope</button>
      <button id="button2">Wildebeest</button>
    </div>

    #div1 {
      display: box;
      box-lines: multiple;
      box-pack: center;
      width: 300px;
    }
    button {
      box-flex: 1.0;
      width: 90px;
      max-width: 90px;
    }

7.1 Multiple Lines and alignment

Note: not sure what this should do yet.

The box-align property may be used to control the alignment of children in multiple line boxes, as with single line boxes. If box-align is start, the first line is placed along the top or left edge of the box (or the bottom or right edge for reverse direction boxes), and additional lines are placed afterward with no extra spacing between them. Each element within a line is placed with that same edge aligned. For instance, in a normal direction, vertical box, the top edges of each element are aligned with the top edge of the line it is within. If box-align is end, the first line is placed along the bottom or right edge of the box (or the top or left edge for reverse direction boxes), and additional lines are placed before. Each element within a line is placed with that same edge aligned.

If box-align is center, the extra spacing is divided evenly on both sides of the box, and the lines placed between this space. Elements are centered within their lines. If box-align is baseline, the elements in a line are aligned by their baselines as with single line boxes, with space placed before or after each element in the line as necessary. The first line is placed as with the value start.

If box-align is stretch, then each line is stretched equally, and the elements within each line are stretched to the size of the largest item in that line.

8 Relationship to Inlines and blocks

A box placed in a block is considered a block level element and will be positioned and sized according to the rules for CSS 2.1 (section 10.3 for widths). Specifically, the following equation applies for calculating the width:

‘margin-left’ + ‘border-left-width’ + ‘padding-left’ + ‘width’ + ‘padding-right’ + ‘border-right-width’ + ‘margin-right’ + scrollbar width (if any) = width of containing block

All children of ‘box’ elements are block-level. If inline-level elements are nested inside boxes, they get wrapped in an anonymous block which then takes part in the box layout.

If the width of a child of a flexible box is computed as ‘auto’, then the used value is the shrink-to-fit width. This calculation is done similarly to how the width of a floating element is determined. Otherwise if the width has a computed value, then that value is used as the width.

In a horizontally oriented box, the flexibility is then applied which may increase or decrease the used width. In a vertically oriented box, the position and size of the child may be adjusted by the value of the container's ‘box-align’ property.

The ‘min-width’, ‘min-height’, ‘max-width’, and ‘max-height’ CSS properties act the same on boxes as on blocks.

The height of a child of a flexible box is determined in the same manner as specified in the CSS specification. The used height may further be adjusted by the container's ‘box-align’ property in a horizontally oriented box, or the flexibility in a vertically oriented box.

Note that it is possible for a horizontally oriented box that the height of a flexible child containing an inline descendant will change when the flexibility is applied. Similarly, in a vertically oriented box, the height of a child may change based on the ‘box-align’ property of the box.

Note: more details are needed here, and the relationship with other CSS constructs is yet to be defined.

Acknowledgments

[This section will contain further acknowledgments.]

Thanks for feedback from Shinichiro Hamaji.

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 23 April 2009. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2009/CR-CSS2-20090423/

Index

• box-align, 4
• box-direction, 3
• box-flex, 5
• box-flex-group, 5
• box-lines, 7
• box-ordinal-group, 3
• box-orient, 2
• box-pack, 6
• flex groups, 5
• flexible, 5
• inflexible, 5
• ordinal groups, 3

Property index