Title: CSS Page Floats
TR: https://www.w3.org/TR/css-page-floats-3/
Status Text: Some of the features in this draft were previously published in CSS Generated Content for Paged Media Module [[CSS3GCPM]].
Shortname: css-page-floats
Level: 3
Group: csswg
Status: ED
Work Status: Exploring
ED: https://drafts.csswg.org/css-page-floats/
Previous Version: https://www.w3.org/TR/2015/WD-css-page-floats-3-20150915/
Editor: Johannes Wilm, Vivliostyle, johanneswilm@vivliostyle.com, w3cid 76051
Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net, w3cid 43241
Editor: Rachel Andrew, Google, rachelandrew@google.com, w3cid 81117
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, inline, max-width, max-height

Overview

Page 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 CSS Page Floats should probably be replaced with a more appropriate name.

Value Definitions

Terminology

Float

An element which has float set to something else than none.

Inline float

A float which has float set to inline.

Page float

A float which has float set to something else than inline or none.

Float block formatting context

The block formatting context which is generated by a float and which contains its contents.

Float anchor

The float anchor is the point in the flow where the float had appeared had it not been a float and instead had been an empty inline element with no margins, borders or padding.

Float containing block formatting context

The block formatting context inside of which an float is embedded.

Initial float reference

The entity to which the float is aligned initially, before float placement takes place.

Float reference

The entity to which the float is aligned.

Not overlapping

Two elements are not overlapping if the margin box of one element is not overlapping the margin box of the other element.

Floating to the inline-start/inline-end and block-start/block-end

Floating elements can float to the start or end of the float anchor's line or block, specified by the 'float' attribute. The floats are aligning to the start or end of a float reference, specified by the 'float-reference' attribute. The float reference can be the float anchor's line box, column, region or page.

The 'float-reference' property

    Name: float-reference
    Value: inline | column | region | page
    Initial: inline
    Applies to: all elements.
    Inherited: no
    Percentages: N/A
    Computed value: specified keyword
    Animation type: discrete
  

inline

The float reference is the line box of the float anchor. The float containing block formatting context is the same as that of the float anchor. The float is an inline float.

column

The float reference is the column in a multi column environment in which the float anchor is placed. If the float anchor is not inside a column, the float reference is the line box of the float anchor. The float containing block formatting context is a new block formatting context with the same dimensions and placement as the float reference. The float is a page float.

region

The float reference is the region in a region-chain within which the float anchor is placed. If the float anchor is not inside a region, the float reference is the line box of the float anchor. The float containing block formatting context is a new block formatting context with the same dimensions and placement as the float reference. The float is a page float.

page

The float reference of the float is the page within which the float anchor is placed. If the float anchor is not inside a page, the float reference is the line box of the float anchor. The float containing block formatting context is a new block formatting context with the same dimensions and placement as the float reference. The float is a page float.

<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>
    

The 'float' property

  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
  Computed value: as specified
  Animation type: by computed value type

inline-start

If the float reference is a line box, the element generates a box that is floated to the line-start outer edge of the float reference and content flows on the line-end side of the box. If the float reference is not a line box, the element generates a box that is floated to the line-start and block-start outer edges of the float reference.

inline-end

If the float reference is a line box, the element generates a box that is floated to the line-ebd outer edge of the float reference and content flows on the line-start side of the box. If the float reference is not a line box, the element generates a box that is floated to the line-end and block-end outer edges of the float reference.

block-start

If the float reference is a line box, block-start behaves like inline-start. If the float reference is not a line box, the element generates a box that is floated to the block-start and line-start outer edges of the float reference. The initial value of the max-width or max-height property that refers to the inline size of the float is '100%'. Content flows on the block-end side of the box.

block-end

If the float reference is a line box, block-end behaves like inline-end. If the float reference is not a line box, the element generates a box that is floated to the block-end and line-end outer edges of the float reference. The initial value of the max-width or max-height property that refers to the inline size of the float is '100%'. Content flows on the block-start side of the box.

left

If the float reference is a line box, behaves like inline-start or inline-end, whichever corresponds to line-left for the float reference. Otherwise, behaves like block-end, inline-start or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

right

If the float reference is a line box, behaves like inline-start or inline-end, whichever corresponds to line-right for the float reference. Otherwise, behaves like block-start, inline-start or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

top

Behave like block-start or inline-start depending on the float containing block’s 'direction' and 'writing-mode'.

bottom

Behave like block-end or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

snap-block()

The ''snap-block()'' function is defined as:

        snap-block() = snap-block( <> , [ start | end | near ]? )
      

Has no effect if the element is an inline float. If the element is a page float, it 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.

snap-block

Behaves as snap-block(2em, near)

snap-inline()

The ''snap-inline()'' function is defined as:

        snap-inline() = snap-inline( <> , [ left | right | near ]? )
      

Has no effect if the element is an inline float. If the element is a page float, it 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.

snap-inline

same as snap-inline(2em, near)

none

The box is not floated.

Float figure to top of reference column:

.figure { float-reference: column; float: top }

In this example a block-start float that does not fill the entire inline size of the float reference is placed at the start of the block and line.

.figure { float-reference: column; float: block-start; width: 50% }

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. To avoid the white space, the image can be floated to the nearest edge (in the block direction):

.figure { float-reference: column; float: snap-block }

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:

In this example, two figures naturally appear in the text flow: A typographer would typically try to avoid single lines of text above/below figures, which can be achieved with:

div.figure { float-reference: column; float: snap-block(1.5em) }

The length value specifies the reach of the snap function; in this example the second figure is affected, but not the first.

In this example, two figures naturally appear in the text flow: To make the figures snap to the nearest edges, this code can be applied:

div.figure { float-reference: column; float: snap-block(2.5em) }

The resultant rendering is:

table { float: snap }
table { float: snap-block(3em) }
table { float: snap-block(3em, bottom) }
table { float: snap-block(3em 2em, bottom) }

The 'clear' property

    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
    Computed value: specified keyword
    Animation type: discrete
  

inline-start

If applied to an inline float, 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 page float, the float reference in which the page float is placed will be seen as full when determining whether it can host subsequent page floats that float in the inline-start direction.

inline-end

If applied to a block-level element or an inline float, 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 page float, the float reference in which the page float is placed will be seen as full when determining whether it can host subsequent page floats that float in the inline-end direction.

block-start

If applied to a block-level element or an inline float, behaves like inline-start. If applied to a page float, the float reference in which the page float is placed will be seen as full when determining whether it can host subsequent page floats that float in the block-start direction.

block-end

If applied to a block-level element or an inline float, behaves like inline-end. If applied to a page float, the float reference in which the page float is placed will be seen as full when determining whether it can host subsequent page floats that float in the block-end direction.

left

Behave like block-end, inline-start or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

right

Behave like block-start, inline-start or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

top

Behave like block-start or inline-start depending on the float containing block’s 'direction' and 'writing-mode'.

bottom

Behave like block-end or inline-end depending on the float containing block’s 'direction' and 'writing-mode'.

both-inline

Behave like inline-start and inline-end.

both-block

Behave like block-start and block-end.

both

Behave like both-inline.

all

Behave like both-block and both-inline.

In this example, the two figures may appear in the same column:

.figure { float-reference: column; float: bottom; clear: none }

<div class=figure></div>
<div class=figure></div>

In this example, the two figures will appear in different columns:

.figure { float-reference: column; float: bottom; clear: bottom }

<div class=figure></div>
<div class=figure></div>

In this example, the two figures may appear at the bottom of the same column due to clearing only at the top:

.figure { float-reference: column; float: bottom; clear: top }

<div class=figure></div>
<div class=figure></div>

In this example, the two figures will appear in different columns due to clearing at the bottom:

.figure { float-reference: column; float: bottom; clear: bottom }

<div class=figure></div>
<div class=figure></div>

.figure { float-reference: page; float: top; clear: top }

<div class=figure></div>
<div class=figure></div>

.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>

Deferring page floats

The 'float-defer' property

      Name: float-defer
      Value: <integer> | last | none
      Initial: none
      Applies to: floats
      Inherited: no
      Percentages: N/A
      Computed value: specified keyword or integer
      Animation type: discrete
    

none

The initial float reference is the fragmentation container in which the float anchor is placed after all previous page floats have been placed.

<integer>

A positive integer value indicates that the initial float reference of the page float should be Nth fragmentation container of the `fragmentation flow`, where N is the value of the `float-defer` property plus the order number of the fragmentation container in which the float anchor is placed after all previous page floats have been placed within the given fragmentation context. If N is larger than the order number of the last fragmentation container within the given fragmentation context at the time of assignment, then N is the order number of the last fragmentation container within the given fragmentation context. A negative integer value indicates that the initial float reference of the page float should be a fragmentation container of the fragmentation context, counting backward from the end, so that -1 is the last fragmentation container, -2 is the next-to-last, etc. . In the case of a negative integer value, the initial float reference is the Nth fragmentation container of the fragmentation context, where N is 1 plus the order number of the last fragmentation container within the given fragmentation context after all previous page floats have been placed plus the value of the `float-defer` property. Zero is the same as `none`. If the value of the `float-defer` property would cause the initial float reference to be an inexistent fragmentation container, the property is interpreted as if it were zero.

Negative float-defer values put the initial float reference a certain number to be a certain amount of fragmentation containers from the last fragmentation container at the time of the placement. Subsequent page float stacking can mean that a page float is being placed in a later fragmentation container (a page float with float-defer set to -3 can end up being placed in the last fragmentation container), and later page floats may mean that new fragmentation containers are added, so that the a fragmentation container that previously was Nth last fragmentation container within a fragmentation context now is the N+Xth last. Additional fragmentation container(s) that are added after the page float was placed, will not cause the page float to be moved.

last

The initial float reference is the last fragmentation container within the given fragmentation context after all previous page floats have been placed.

.figure { float-reference: region }
.figure { float: top }
.figure { float-defer: 1 }

Float figure to the top of the next-to-last column:

.figure { float-reference: column; float: top; float-defer: -2 }

.figure { float-reference: page }
.figure { float: top }
.figure { float-defer: -1 }

.figure { float-reference: column }
.figure { float: top }
.figure { float-defer: last }

Float figure to top of the last column:

.figure { float-reference: column; float: top; float-defer: last }

Wrapping around page floats

The 'float-offset' property

    Name: float-offset
    Value: <> | <>
    Initial: 0
    Applies to: floats
    Inherited: no
    Percentages: see prose
    Computed value: computed <> value
    Animation type: by computed value type
  

<>

Percentage values are computed according to this formula:

      (containing-block-width - float-width) * percentage
      (containing-block-height - float-height) * percentage
    

img {
  float-reference: column;
  float: left;
  float-offset: 2em;
}

img {
  float-reference: column;
  float: right;
  float-offset: 5px;
}

Page float placement

1. All page floats with 'float-reference' set to `page` are placed, in document order, before those with 'float-reference' set to `region` and `column`.
2. Thereafter, page floats with 'float-reference' set to `column` and `region` are placed in document order.

1. Determine the initial float reference by considering the fragmentation container in which the float anchor is placed and the `float-defer` property of the page float. The float reference is initially set to be the same as the initial float reference.
2. Determine if the given float reference has enough space or can be expanded to host the page float, if the rules of float stacking and float reference growth are to be followed. If this is not the case, and the float reference is not the last fragmentation container within the given fragmentation context, then make the following fragmentation container within the given fragmentation context the float reference. Repeat this step until the float reference can be expanded enough to host the page float or it is the last fragmentation container within the given fragmentation context.
3. If the float reference is the last fragmentation container within the given fragmentation context, and it has not enough space and cannot be expanded to host the page float, then do the following:
1. If the fragmentation context allows for the addition of another fragmentation container and an additional fragmentation container would have the needed size to host the page float, a new fragmentation container is added to the end of the fragmentation context. The float reference is set the newly created fragmentation container.
2. Otherwise, if the fragmentation container is a region, then the 'regionOverset` attribute of the fragmentation container is set to `overset`.
4. The page float is placed in the float reference according to the rules of 'float stacking' and 'float reference growth'.

Float reference growth

Rules for page float stacking

We can effectively currently float to two corners: inline-start/block-start and inline-end/block-end. We should augment this with the option to have a secondary float direction to allow basic 2D floating.

Floats and absolutely positioned exclusions

Differences between inline floats and absolutely positioned elements

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.

However, in the case of inline floats, the block formatting context that contains them (the float containing block formatting context) is required to include the area occupied by the float, which is not a requirement for absolutely positioned elements.

An inline float inside a float containing block formatting context given by a display-inline-block element. The element, which has a green border, is expanded to include the brown float.

<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>

In comparison, the below is the same HTML, but the float is replaced by an absolutely positioned element that is also an exclusion. The float containing block formatting context 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.

<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>

Overconstrained floats

.figure { float-reference: column; float: top; float-defer: 1000 }

.figure { float-reference: column; float: top; float-defer: -5 }

p { float-reference: page; float: top; float-defer: last }

.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>

.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>

1. honor 'clear: top/bottom'
2. honor 'float-defer'
3. honor 'float: top/bottom'
4. display all content (as described by other CSS properties)
5. keep the number of pages to a minimum

Acknowledgments