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<h1>CSS Page Floats</h1>
<pre class='metadata'>
Shortname: css-page-floats
Level: 3
Group: csswg
Status: ED
ED: http://dev.w3.org/csswg/css-page-floats/
Previous Version: https://hg.csswg.org/drafts/raw-file/108d7e3ff204/css-page-floats/Overview.html
Editor: Johannes Wilm, Vivliostyle, johanneswilm@vivliostyle.com
Former Editor: Håkon Wium Lie, Opera Software, howcome@opera.com
Abstract: This document describes floats that move to the top or bottom of content
Abstract: passages. This feature has traditionally been used in print
Abstract: publications in which figures and photos are moved to the top or bottom
Abstract: of columns or pages, along with their captions.
Abstract: This draft describes how to achieve this effects for floats within pages,
Abstract: columns, regions and elements.
Ignored Terms: near, 3em, intrude, top-corner, bottom-corner, left, right, both, element, none, floats, float-stacking-order, inline, max-width, max-height
</pre>
<h2 id="overview">Overview</h2>
This specification adds new keywords on the 'float' property.
This document allows to specify whether a <a>float</a> floats to align with a
<a>float reference</a> inline box, column, region or page. In the case of floats
with a reference column, region or page, placement can be deferred to a
subsequent column, region or page with the <em>float-defer</em> properties.
Floats with a column reference can be set to span several columns, thereby
supporting commonly used newspaper layouts. New values on the 'clear' property
add further ways of refining layouts.
The way contents wrap around floats can be controlled by changing the value of
the 'wrap-flow' property which initially is set to 'both' for floats.
<p class="issue">
Floats as defined here work with different types of fragmentation types
(columns, regions, pages) as well as container elements. The specification
is no longer specific to print or to pages.
At the same time, inline floats and page floats differ in many ways, and it may
(or may not) be a good idea to separate the two entirely.
Therefore, the name <em>CSS Page Floats</em> should probably be replaced with
a more appropriate name.
</div>
<h2 id="terms">
Terminology</h2>
<dl export>
<dt><dfn>Float</dfn>
<dd>
An element which has float set to something else than none.
<dt><dfn>Float block formatting context</dfn>
<dd>
The block formatting context which is generated by a float and which contains
its contents.
<dt><dfn>Float anchor</dfn>
<dd>
The float anchor is the point in the flow where the float had appeared had
it not been a float.
<dt><dfn>Float containing block formatting context</dfn>
<dd>
The block formatting context inside of which the float is embedded.
<dt><dfn>Float reference</dfn>
<dd>
The entity to which the float is aligned.
</dl>
<h2 id="positioning">
Difference between inline floats and absolutely positioned elements</h2>
<p>
Inline floats and absolutely positioned elements are both out-of-flow elements.
Absolutely positioned elements that are also exclusions can imitate many of
the features of floats.
<p>
However, in the case of inline floats, the block formatting context that
contains them (the <a>float containing block formatting context</a>) is
required to include the area occupied by the float, which is not a requirement
for absolutely positioned elements.
<div class="example">
An inline float inside a <a>float containing block formatting context</a> given
by a display-inline-block element. The element, which has a green border, is
expanded to include the brown float.
<code><pre>
<style>
.float {
float: left;
margin: 5px;
}
.border {
border: 3px solid black;
margin: 5px;
}
#outer {
border: 1px solid green;
display: inline-block;
}
canvas {
background-color: brown;
}
p {
margin: 5px;
}
</style>
<div id="outer">
<p class="border">
<span class="float border">
<canvas width="100" height="100"/>
</span>
First paragraph.
</p>
<p class="border">
Second paragraph and some more text.
</p>
</div>
</pre></code>
<img alt="sample rendering" src="images/float_containing_context.png">
In comparison, the below is the same HTML, but the float is replaced by an
absolutely positioned element that is also an exclusion. The <a>float
containing block formatting context</a> is still given by a
display-inline-block element. However, the element, marked by a green border,
does not expand to include the brown, absolutely positioned element.
<code><pre>
<style>
.float {
position: absolute;
top: 8px;
left: 8px;
wrap-flow: both;
}
.border {
border: 3px solid black;
margin: 5px;
}
#outer {
border: 1px solid green;
display: inline-block;
position: relative;
}
canvas {
background-color: brown;
}
</style>
<div id="outer">
<p class="border">
<span class="float border">
<canvas width="100" height="100"/>
</span>
First paragraph.
</p>
<p class="border">
Second paragraph and some more text.
</p>
</div>
</pre></code>
<img alt="sample rendering" src="images/positioned_containing_context.png">
</div>
<h2 id="floating">
Floating to the inline-start/inline-end and block-start/block-end</h2>
<p>
Floating elements can float to the start or end of the <a>float anchor</a>'s line or
block, specified by the 'float' attribute. The floats are aligning to the start
or end of a <a>float reference</a>, specified by the 'float-reference' attribute. The
<a>float reference</a> can be the <a>float anchor</a>'s line box, column, region or page.
<h3 id="float-reference-property">The 'float-reference' property</h3>
<pre class="propdef">
Name: float-reference
Value: inline | column | region | page
Initial: inline
Applies to: all elements.
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified.
Animatable: no
</pre>
<dl dfn-for="float-reference" dfn-type="value">
<dt><dfn>inline</dfn>
<dd>
The <a>float reference</a> is the line box of the <a>float anchor</a>.
The <a>float containing block formatting context</a> is the same as that of the
<a>float anchor</a>.
<dt><dfn>column</dfn>
<dd>
The <a>float reference</a> is the column in a multi column environment in
which the <a>float anchor</a> is placed. If the <a>float anchor</a> is not
inside a column, the <a>float reference</a> is the line box of the <a>float
anchor</a>.
The <a>float containing block formatting context</a> is a new block formatting
context with the same dimensions and placement as the <a>float reference</a>.
<dt><dfn>region</dfn>
<dd>
The <a>float reference</a> is the region in a region-chain within which the
<a>float anchor</a> is placed. If the <a>float anchor</a> is not inside a
region, the <a>float reference</a> is the line box of the <a>float
anchor</a>.
The <a>float containing block formatting context</a> is a new block formatting
context with the same dimensions and placement as the <a>float reference</a>.
<dt><dfn>page</dfn>
<dd>
The <a>float reference</a> of the float is the page within which the <a>float
anchor</a> is placed. If the <a>float anchor</a> is not inside a page, the
<a>float reference</a> is the line box of the <a>float anchor</a>.
The <a>float containing block formatting context</a> is a new block formatting
context with the same dimensions and placement as the <a>float reference</a>
</dl>
<div class="issue">
The <a>float containing block formatting context</a> having the same dimensions
as the <a>float reference</a> means that all floats that are not inline floats
cannot move outside of their float references. Is this wanted?
</div>
<div class="issue">
It is not possible to directly specify which element to use as the reference
element. This may be of interest to have.
Maybe something like:
<code><pre>
<style>
.float {
float-reference: float-container;
}
#container {
float-container: true;
}
</style>
<div id="container">
<p>First paragraph<span class="float">FLOAT</span></p>
<p>Second paragraph
<span class="inline-block">[<span class="float">FLOAT</span>] </span>
And some more text</p>
</div>
</pre></code>
This should float both floats with reference to the <div id="container">
element, rather than the <P> and inline <SPAN> elements.
</div>
<h3 id="float-property">The 'float' property</h3>
<pre class="propdef">
Name: float
Value: block-start | block-end | inline-start | inline-end | snap-block | snap-inline | left | right | top | bottom | none
Initial: none
Applies to: all elements.
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified.
Animatable: no
</pre>
<dl dfn-for="float" dfn-type="value">
<dt><dfn>inline-start</dfn>
<dd>
The element generates a box that is floated to the line-start outer edge of
the <a>float reference</a>. Content flows on the line-end side of the box
(subject to the ‘clear’ property).
<dt><dfn>inline-end</dfn>
<dd>
The element generates a box that is floated to the line-end outer edge of the
<a>float reference</a>. Content flows on the line-start side of the box
(subject to the ‘clear’ property).
<dt><dfn>block-start</dfn>
<dd>
If the <a>float reference</a> is a line box, block-start behaves like
inline-start.
If the <a>float reference</a> is not a line box, the element generates a box
that is floated to the block-start and line-start outer edges of the <a>float
reference</a>.
The initial value of the <a>max-width</a> or <a>max-height</a> property that
refers to the <a>inline size</a> of the float is '100%'.
Content flows on the block-end side of the box (subject to the ‘clear’
property).
<dt><dfn>block-end</dfn>
<dd>
If the <a>float reference</a> is a line box, block-end behaves like inline-end.
If the <a>float reference</a> is not a line box, the element generates a box
that is floated to the block-end and line-start outer edges of the <a>float
reference</a>.
The initial value of the <a>max-width</a> or <a>max-height</a> property that
refers to the <a>inline size</a> of the float is '100%'.
Content flows on the block-start side of the box (subject to the ‘clear’
property).
<div class="issue">
Should block-end and block-start floats be allowed to have an inline-size that
is larger than the that of the float reference? What should happen in that
case?
</div>
<dt><dfn>left</dfn>
<dd>
Behave like block-end, inline-start or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>right</dfn>
<dd>
Behave like block-start, inline-start or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>top</dfn>
<dd>
Behave like block-start or inline-start depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>bottom</dfn>
<dd>
Behave like block-end or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>snap-block(<length> <length>? [, start | end | near ]?)</dfn>
<dd>
Makes the element float to the start or the end of the block if it
naturally appears within a certain distance from either one. The length
value(s) specifies the maximum distance from the start/end that an element
must be within in order to be floated; one length value specifies the
distance from both the start and the end, two length values specify the
distance from the start and end, respectively.
The optional keyword value specifies where the element is floated: start,
end, or the nearest of the two. The initial value is 'near'. If 'near'
is in effect and the element is within the specified distance both from
the start and the end, end wins.
An element is considered to be a float if it has a snap-block() value,
even if the element does not appear within the specified distance. This
way, it can be determined whether an element is float or not without
laying out the document.
<dt><dfn>snap-block</dfn>
<dd>same as <tt>snap-block(2em, near)</tt>
<dt><dfn>snap-inline(<length> <length>? [, left | right | near ]?)</dfn>
<dd>
Makes the element float to the line start or line end if it naturally
appears within a certain distance from the start or end of the line. The
length value(s) specifies the maximum distance from the start/end that an
element must be within in order to be floated; one length value specifies
the distance from both the start and the end, two length values specify the
distance from the start and end, respectively.
The optional keyword value specifies where the element is floated: line
start, line end, or the nearest of the two. The initial value is 'near'.
If 'near' is in effect and the element is within the specified distance
both from the start and the end, end wins.
An element is considered to be a float if it has a snap-inline() value,
even if the element does not appear within the specified distance. This
way, it can be determined whether an element is float or not without laying
out the document.
<dt><dfn>snap-inline</dfn>
<dd>same as <tt>snap-inline(2em, near)</tt>
<dt><dfn>none</dfn>
<dd>The box is not floated.
</dl>
<div class="issue">
There is currently no way to float into a combination of directions (top right,
right top, left bottom, bottom left, etc.).
</div>
<div class="example">
Float figure to top of reference column:
<pre>
.figure { float-reference: column; float: top }
</pre>
<img alt="sample rendering" src="images/7.png">
</div>
<div class="example">
In this example a block-start float that does not fill the entire <a>inline size</a>
of the float reference is placed at the start of the block and line.
<pre>
.figure { float-reference: column; float: block-start; width: 50% }
</pre>
<img alt="sample rendering" src="images/block_line_start.png">
</div>
<div class="example">
In this example, a figure naturally appears close to a column break. There is
not enough space for the figure in the first column, and it is therefore placed
in the second column, leaving white space at the bottom of the first column.
<img alt="sample rendering" src="images/23.png">
To avoid the white space, the image can be floated to the nearest edge (in the
block direction):
<pre>
.figure { float-reference: column; float: snap-block }
</pre>
In this example, the figure is already at the nearest edge, so it does not
move. However, floats allow subsequent content to be displayed before the
float and the white space can therefore be filled:
<img alt="sample rendering" src="images/7.png">
</div>
<div class="example">
In this example, two figures naturally appear in the text flow:
<img alt="sample rendering" src="images/20.png">
A typographer would typically try to avoid single lines of text above/below
figures, which can be achieved with:
<pre>
div.figure { float-reference: column; float: snap-block(1.5em) }
</pre>
The length value specifies the reach of the snap function; in this example the
second figure is affected, but not the first.
</div>
<div class="example">
In this example, two figures naturally appear in the text flow:
<img alt="sample rendering" src="images/20.png">
To make the figures snap to the nearest edges, this code can be applied:
<pre>
div.figure { float-reference: column; float: snap-block(2.5em) }
</pre>
The resultant rendering is:
<img alt="sample rendering" src="images/22.png">
</div>
<div class="example">
In this example, tables will snap to the top/bottom if the top/bottom of the
border box is closer than '3em' from the top/bottom of the float-reference
which is a block element.
<pre>
table { float: snap }
table { float: snap-block(3em) }
table { float: snap-block(3em, bottom) }
table { float: snap-block(3em 2em, bottom) }
</pre>
</div>
<div class="note">
More examples with regions and elements as <a>float reference</a>s needed.
</div>
<h2 id="stacking_floats">
Stacking floats</h2>
Floats for any given line/column/region/page are placed in the order they appear
in the source document. This means that floats that have been deferred from
previous columns/regions/pages will be placed first, and thereafter floats from
the current line/column/region/page.
Floats with the same <a>float reference</a> are made to never overlap. If the
<a>float reference</a> is a column, region or page and the float containing
context is limited in size, and there is not enough space to draw a particular
float, that float is automatically deferred to the next column/region/page.
The stacking order of the floats within a related to a particular <a>float
reference</a> are determined by the block flow and inline directions.
<p class="note">
Detailed instructions and examples for float placement are needed, including
how top/bottom and left/right floats interact, and how floats with different
float references interact.
<p class="note">
Examples and property definition of 'float-stacking-order' needed! Such as "The
first float with the float property set to a particular value is floated as the
outermost element. The second as the second outermost element, etc. ."
<h2 id="clearing_page_floats">
The 'clear' property</h2>
<pre class="propdef">
Name: clear
Value: inline-start | inline-end | block-start | block-end | left | right | top | bottom | none
Initial: none
Applies to: block-level elements, floats, regions, pages
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified.
Animatable: no
</pre>
To prevent stacking of floats, the 'clear' property can be used:
<dl dfn-for="clear" dfn-type="value">
<dt><dfn>inline-start</dfn>
<dd>
If applied to a block-level element or a float with a 'float-reference' set
to inline, requires that the block-start outer edge of the box comes after
the block-end outer edge of any inline-start-floats with an
inline-start-float-reference that resulted from elements earlier in the
source document.
If applied to a float with a 'float-reference' set to column/region/page, it
requires that the block-start outer edge of the box follows the block-end
outer edge of any inline-start-floats with a inline-start-float-reference
that resulted from elements earlier in the source document. If the <a>float
reference</a>'s height is limited and there is insufficient space to place
this float, it requires that this and all further inline-start-floats
relating to the same <a>float reference</a> are deferred to the next
page/column/region.
<dt><dfn>inline-end</dfn>
<dd>
If applied to a block-level element or a float with a 'float-reference' set
to inline, requires that the block-start outer edge of the box comes after
the block-end outer edge of any inline-end-floats with an
inline-end-float-reference that resulted from elements earlier in the
source document.
If applied to a float with a 'float-reference' set to column/region/page, it
requires that the block-start outer edge of the box follows the block-end
outer edge of any inline-end-floats with a inline-end-float-reference that
resulted from elements earlier in the source document. If the <a>float
reference</a>'s height is limited and there is insufficient space to place
this float, it requires that this and all further inline-end-floats
relating to the same <a>float reference</a> are deferred to the next
page/column/region.
<dt><dfn>block-start</dfn>
<dd>
If applied to a block-level element or a float with a 'float-reference' set
to inline, behave like inline-start.
If applied to a float with a 'float-reference' set to column/region/page,
requires that this and all further block-start-floats relating to the same
<a>float reference</a> are deferred to the next page/column/region.
<dt><dfn>block-end</dfn>
<dd>
If applied to a block-level element or a float with a 'float-reference' set
to inline, behave like inline-end.
If applied to a float with a 'float-reference' set to column/region/page,
requires that this and all further block-end-floats relating to the same
<a>float reference</a> are deferred to the next page/column/region.
<dt><dfn>left</dfn>
<dd>
Behave like block-end, inline-start or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>right</dfn>
<dd>
Behave like block-start, inline-start or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>top</dfn>
<dd>
Behave like block-start or inline-start depending on the float
containing block’s 'direction' and 'writing-mode'.
<dt><dfn>bottom</dfn>
<dd>
Behave like block-end or inline-end depending on the float
containing block’s 'direction' and 'writing-mode'.
</dl>
<div class="example">
In this example, the two figures may appear in the same column:
<pre>
.figure { float-reference: column; float: bottom; clear: none }
<div class=figure></div>
<div class=figure></div>
</pre>
<img src="images/16.png">
</div>
<div class="example">
In this example, the two figures will appear in different columns:
<pre>
.figure { float-reference: column; float: bottom; clear: bottom }
<div class=figure></div>
<div class=figure></div>
</pre>
<img src="images/17.png">
</div>
<div class="example">
In this example, the two figures may appear at the bottom of the same column due to clearing only at the top:
<pre>
.figure { float-reference: column; float: bottom; clear: top }
<div class=figure></div>
<div class=figure></div>
</pre>
<img src="images/16.png">
</div>
<div class="example">
In this example, the two figures will appear in different columns due to clearing at the bottom:
<pre>
.figure { float-reference: column; float: bottom; clear: bottom }
<div class=figure></div>
<div class=figure></div>
</pre>
<img src="images/17.png">
</div>
<div class="example">
In this example, the two figures end up the top corner of two different pages:
<pre>
.figure { float-reference: page; float: top; clear: top }
<div class=figure></div>
<div class=figure></div>
</pre>
</div>
<div class="example">
In this example, the two figures request different positions, and they may
therefore end up in the same column:
<pre>
.figure.one { float-reference: column; float: top; clear: top }
.figure.two { float-reference: column; float: bottom; clear: bottom }
<div class="figure one"></div>
<div class="figure two"></div>
</pre>
</div>
<h2 id="deferring_floats">Deferring floats</h2>
A float can be deferred to a following column/region/page, depending on its
<a>float reference</a>. Floats with 'float-reference' set to 'inline' cannot be
deferred.
Specified float deferring takes place before float stacking. Float stacking can
lead to floats being deferred to the next column/region/page if the
column/region/page they were targeted for originally lacks the required space.
This new property is introduced to control deferring floats:
<h3 id="float-defer-property">The 'float-defer' property</h3>
<pre class="propdef">
Name: float-defer
Value: <integer> | last | none
Initial: none
Applies to: floats
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified.
Animatable: no
</pre>
This property specifies whether a floats should appear in their natural
column/region/page, or in a following column/region/page. Values are:
<dl>
<dt>none
<dd>the page float appears in the natural column/region/page
<dt><integer>
<dd>
A positive integer value indicates that the page float should be
displayed in a following column/region/page. A negative integer value
indicates that the page float should be displayed in a following
column/region/page, counted from the last column/region/page. Zero is not
a legal value.
<div class="example">
Float figure to the top of the column that follows the natural column:
<pre>
.figure { float-reference: column }
.figure { float: top }
.figure { float-defer: 1 }
</pre>
</div>
<div class="example">
Float figure to the top of the next-to-last column:
<pre>
.figure { float-reference: column; float: top; float-defer: -1 }
</pre>
<img alt="sample rendering" src="images/7.png">
</div>
<div class="example">
Float figure to top of the last column of the multicol element on the current page:
<pre>
.figure { float-reference: column; float: top; float-defer: last }
</pre>
<img alt="sample rendering" src="images/6.png">
</div>
<div class="example">
Float figure to the top of the first column on the next-to-last page:
<pre>
.figure { float-reference: page }
.figure { float: top }
.figure { float-defer: -1 }
</pre>
</div>
<dt>last
<dd>
The page float should be displayed in the last colum, region or page.
<div class="example">
Float figure to the top of the last column:
<pre>
.figure { float-reference: column }
.figure { float: top }
.figure { float-defer: last }
</pre>
</div>
<dt>none
<dd>The floats should appear in their natural column/region/page, if possible.
</dl>
Zero is not a legal value.
<h2 id="wrapping-around-page-floats">Wrapping around floats</h2>
Floats have their 'wrap-flow' property set to 'both' initially and are treated
like exclusions. This specification does not make any further specification about
wrapping contents around floats.
<div class="issue">
Should the 'wrap-flow' really be set to both, or should the flow be restricted
to only be on one side?
</div>
<h2 id="the-float_offset-property">The 'float-offset' property</h2>
<pre class="propdef">
Name: float-offset
Value: <length> <length> ?
Initial: 0
Applies to: floats
Inherited: no
Percentages: N/A
Media: visual
Computed value: one or two absolute lengths
Animatable: no
</pre>
This property pushes a float in opposite direction of the where it has been
floated with 'float'.
This property can only influence a float along an axis it has been floated.
<div class="example">
<pre>
img {
float-reference: column;
float: left;
float-offset: 2em;
}
</pre>
In this example, the image is floated to the left. Therefore, 'float-offset'
may only push the element to the right.
</div>
<div class="example">
<pre>
img {
float-reference: column;
float: right;
float-offset: 5px;
}
</pre>
<img alt="sample rendering" src="images/14.png">
</div>
Percentage values are computed according to this formula:
<pre>
(containing-block-width - float-width) * percentage
(containing-block-height - float-height) * percentage
</pre>
<div class="example">
Pull quotes are often centered in a column. In this example, the pull quote is
floated to the right, and then pushed back into the center.
<img width=213 src="images/exclusion_wrap_side_left.png">
<pre>
.pullquote {
float-reference: region;
float: right;
float-offset: 50%; /* 50% centers the box */
}
</pre>
<p class="note">
The body text in the example should wrap around both sides of the floated
element to follow this specification.
</div>
<h2 id="overconstrained-page-floats">Overconstrained floats</h2>
In many cases, the specified values on these properties cannot be honored.
<div class="example">
The number of columns is limited, and high values therefore cannot be honored:
<pre>
.figure { float-reference: column; float: top; float-defer: 1000 }
</pre>
</div>
<div class="example">
A narrow screen may only have room for one column, in which case this request
cannot be honored:
<pre>
.figure { float-reference: column; float: top; float-defer: -5 }
</pre>
</div>
<div class="example">
In long documents, all content cannot fit on the last page, and this rule
therefore cannot be honored:
<pre>
p { float-reference: page; float: top; float-defer: last }
</pre>
</div>
Floats are processed in the order they appear in the source. However, the
visual order of floats may not be the same as the source order.
<div class="example">
Consider this code:
<pre>
.one { float-reference: page; float: top; float-defer: last }
.two { float-reference: column; float: top; clear: column }
<div class=one></div>
<div class=two></div>
</pre>
In this example, the first element requests to appear on the last page, while
the second element requests to appear in the natural column. If the natural
column of the second element appears on a page before the last page, the second
element will appear visually before the first.
</div>
<div class="example">
Consider this code:
<pre>
.one { float-reference: page; float: top; float-defer: last }
.two { float-reference: page; float: top; clear: page; }
<div class=one></div>
<div class=two></div>
</pre>
If all content can fit on one page, the first page will also be the last page.
The first element is processed first and is placed on top of the first page.
Then the second element is processed. It requests a clear top, somthing which is
not possible on the first page. Therefore, a second page is created and the
first element is moved there. Even if the first element requests to be on the
last page, it will not appear there.
</div>
When resolving over-constrained layouts, the order of importance for defined
goals are:
<ol>