In CSS Level 1 [[CSS1]], placing more content than would fit inside an element with a specified size was generally an authoring error. Doing so caused the content to extend outside the bounds of the element, which would likely cause that content to overlap with other elements.

CSS Level 2 [[CSS21]] introduced the 'overflow' property, which allows authors to have overflow be handled by scrolling, which means it is no longer an authoring error. It also allows authors to specify that overflow is handled by clipping, which makes sense when the author's intent is that the content not be shown.

However, scrolling is not the only way to present large amounts of content, and may even not be the optimal way. After all, the codex replaced the scroll as the common format for large written works because of its advantages.

This specification introduces a mechanism for Web pages to specify that an element of a page should handle overflow through pagination rather than through scrolling.

This specification also extends the concept of overflow in another direction. Instead of requiring that authors specify a single area into which the content of an element must flow, this specification allows authors to specify multiple fragments, each with their own dimensions and styles, so that the content of the element can flow from one to the next, using as many as needed to place the content without overflowing.

In both of these cases, implementations must break the content in the block-progression dimension. Implementations must do this is described in the CSS Fragmentation Module [[!CSS3-BREAK]].

Types of overflow

CSS uses the term overflow to describe the contents of a box that extend outside that one of that box's edges (i.e., its content edge, padding edge, border edge, or margin edge). The overflow might be described as the elements or features that cause this overflow, the non-rectangular region occupied by these features, or, more commonly, as the minimal rectangle that bounds that region. A box's overflow is computed based on the boxes and styles of the box and of all its descendants whose containing block chain undefined term? includes the box.

In most cases, any of these types of overflow can be computed for any box from the bounds and properties of that box, and from the overflow (of that type) of each of its children. However, this is not always the case; for example, when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on some of the children, their descendants with ''transform-style: preserve-3d'' must also be examined.

Ink overflow

The ink overflow of a box is the part of that box and its contents that creates a visual effect outside of the box's border box.

Since some effects in CSS (for example, the blurs in 'text-shadow' [[CSS3TEXT]] and 'box-shadow' [[CSS3BG]]) do not define what visual extent they cover, the extent of the ink overflow is undefined.

The ink overflow region is the non-rectangular region occupied by the ink overflow, and the ink overflow rectangle is the minimal rectangle whose axis is aligned to the box's axes and contains the ink overflow region. Note that the ink overflow rectangle is a rectangle in the box's coordinate system, but might be non-rectangular in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].

Scrollable overflow

The scrollable overflow of a box is the set of things extending outside of that box's padding edge for which a scrolling mechanism needs to be provided.

The following definition should be rewritten to use the concept of 3D rendering context [[!CSS3-TRANSFORMS]] and related terms, particularly once those concepts stabilize following changes proposed in the CSS WG meeting on the morning of 2014-01-28.

The scrollable overflow of a box is the union of the following things, all adjusted for transforms undefined concept! into the box's coordinate space:

I wrote this definition off the top of my head, so it can't possibly be right. It's missing tons of pieces!

The handling of preserve-3d subtrees here is probably wrong; the elements should probably count only towards the overflow of the element that flattens them.

The scrollable overflow region is the non-rectangular region occupied by the scrollable overflow, and the scrollable overflow rectangle is the minimal rectangle whose axis is aligned to the box's axes and contains the scrollable overflow region. Note that the scrollable overflow rectangle is a rectangle in the box's coordinate system, but might be non-rectangular in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].

Border box overflow

This concept has been proposed for some uses, such as for determining what the 'outline' property goes around, and as the basis of a coordinate system for specifying clips and masks, but it's not clear if it's needed.

The border-box overflow of a box is the union of the box's border edge and the border edges of the box's descendants.

If needed, define more formally, as for scrollable overflow above. (Maybe even share the definitions in an appropriate way!)

The border-box overflow region is the non-rectangular region occupied by the border-box overflow, and the border-box overflow rectangle is the minimal rectangle whose axis is aligned to the box's axes and contains the border-box overflow region. Note that the border-box overflow rectangle is a rectangle in the box's coordinate system, but might be non-rectangular in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].

Overflow properties

The 'overflow-x' property specifies the handling of overflow in the horizontal direction (i.e., overflow from the left and right sides of the box), and the 'overflow-y' property specifies the handling of overflow in the vertical direction (i.e., overflow from the top and bottom sides of the box)

The 'overflow' property is a shorthand property that sets the specified values of both 'overflow-x' and 'overflow-y' to the value specified for 'overflow'.

Name:	overflow-x, overflow-y
Value:	visible \| hidden \| scroll \| auto \| paged-x \| paged-y \| paged-x-controls \| paged-y-controls \| fragments
Initial:	visible
Applies to:	block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	see below
Animatable:	no
Canonical order:	per grammar

Name:	overflow
Value:	visible \| hidden \| scroll \| auto \| paged-x \| paged-y \| paged-x-controls \| paged-y-controls \| fragments
Initial:	see individual properties
Applies to:	block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited:	no
Percentages:	N/A
Media:	visual
Computed value:	see individual properties
Animatable:	no
Canonical order:	per grammar

The computed values of 'overflow-x' and 'overflow-y' are determined from the cascaded values [[!CSS3CASCADE]] based on the following rules:

If one or both of the cascaded values are fragmenting values, then:
1. If one of the cascaded values is one of the fragmenting values and the other is not, then the computed values are the same as the cascaded values.
2. If both of the cascaded values are fragmenting values, then:
  1. for horizontal writing mode [[!CSS3-WRITING-MODES]], the computed value for 'overflow-y' is the cascaded value and the computed value for 'overflow-x' is ''overflow/hidden'', or
  2. for vertical writing mode [[!CSS3-WRITING-MODES]], the computed value for 'overflow-x' is the cascaded value and the computed value for 'overflow-y' is ''overflow/hidden''.
Otherwise, if one cascaded values is one of the scrolling values and the other is ''overflow/visible'', then computed values are the cascaded values with ''overflow/visible'' changed to ''overflow/auto''.
Otherwise, the computed values are as specified.

When the fragmenting values are used, the overflow from the fragments themselves treats the fragmenting value as ''overflow/hidden''. Is this the right behavior? Give example.

[[CSS3-MARQUEE]] describes an 'overflow-style' property, but it has not picked up implementation experience that the working group is aware of. Should this document treat 'overflow-style' as a defunct proposal, or should this document describe the 'overflow-style' property and attempt to revive it, despite that implementations have implemented 'overflow-x' and 'overflow-y' instead?

There are discussions about how overflow, overflow-style, overflow-x and overflow-y should work and interact with each other. Until consensus on this topic is reached, it is not completely clear which of these should be used for paged-x | paged-y | paged-x-controls | paged-y-controls | fragments

Scrolling and hidden overflow

Explain which directions allow scrolling and which don't, as a function of 'direction' (including propagation of 'direction' to the ICB).

Paginated overflow

overflow:paginate or overflow:pages (or paged-x, paged-y, paged-x-controls, paged-y-controls as [[CSS3GCPM]] has?)

The current implementation of paginated overflow uses the 'overflow'/'overflow-x'/'overflow-y' properties rather than the 'overflow-style' property as proposed in the [[CSS3GCPM]] draft (which also matches the [[CSS3-MARQUEE]] proposal). We should probably switch away from 'overflow-style', but that's not 100% clear.

Fragment overflow

This section introduces and defines the meaning of the new ''fragments'' value of the 'overflow' property.

When the computed value of 'overflow' for an element is ''fragments'', and implementations would otherwise have created a box for the element, then implementations must create a sequence of fragment boxes for that element. (It is possible for an element with ''overflow: fragments'' to generate only one fragment box. However, if an element's computed 'overflow' is not ''fragments'', then its box is not a fragment box.) Every fragment box is a fragmentation container, and any overflow that would cause that fragmentation container to fragment causes another fragment box created as a next sibling of the previous one. Or is it as though it's a next sibling of the element? Need to figure out exactly how this interacts with other box-level fixup. Additionally, if the fragment box is also a multi-column box (as defined in [[!CSS3COL]] though it defines multi-column element) any content that would lead to the creation of overflow columns [[!CSS3COL]] instead is flown into an additional fragment box. However, fragment boxes may themselves be broken (due to fragmentation in a fragmentation context outside of them, such as pages, columns, or other fragment boxes); such breaking leads to fragments of the same fragment box rather than multiple fragment boxes. (This matters because fragment boxes may be styled by their index; such breaking leads to multiple fragments of a fragment box with a single index. This design choice is so that breaking a fragment box across pages does not break the association of indices to particular pieces of content.) Should a forced break that breaks to an outer fragmentation context cause a new fragment of a single fragment box or a new fragment box? Should we find a term other than fragment box here to make this a little less confusing?

What if we want to be able to style the pieces of an element split within another type of fragmentation context? These rules prevent ever using ''::nth-fragment()'' for that, despite that the name seems the most logical name for such a feature.

<!DOCTYPE HTML>
<title>Breaking content into
  equal-sized cards</title>
<style>
  .in-cards {
    overflow: fragments;

    width: 13em;
    height: 8em;

    padding: 4px;
    border: medium solid blue;
    margin: 6px;

    font: medium/1.3 Times New
      Roman, Times, serif;
  }
</style>
<div class="in-cards">
  In this example, the text in the div
  is broken into a series of cards.
  These cards all have the same style.
  The presence of enough content to
  overflow one of the cards causes
  another one to be created.  The second
  card is created just like it's the
  next sibling of the first.
</div>

In this example, the text in the
div is broken into a series of
cards. These cards all have the
same style. The presence of
enough content to overflow
one of the cards causes another

one to be created. The second
card is created just like it's the
next sibling of the first.

We should specify that ''overflow: fragments'' does not apply to at least some table parts, and perhaps other elements as well. We need to determine exactly which ones.

This specification needs to say which type of fragmentation context is created so that it's clear which values of the 'break-*' properties cause breaks within this context. We probably want ''break-*: region'' to apply.

This specification needs a processing model that will apply in cases where the layout containing the fragments has characteristics that use the intrinsic size of the fragments to change the amount of space available for them, such as [[CSS3-GRID-LAYOUT]]. There has already been some work on such a processing model in [[CSS3-REGIONS]], and the work done on a model there, and the editors of that specification, should inform what happens in this specification.

Fragment styling

The ::nth-fragment() pseudo-element

The ::nth-fragment() pseudo-element is a pseudo-element that describes some of the fragment boxes generated by an element. The argument to the pseudo-element takes the same syntax as the argument to the :nth-child() pseudo-class defined in [[!SELECT]], and has the same meaning except that the number is relative to fragment boxes generated by the element instead of siblings of the element.

Selectors that allow addressing fragments by counting from the end rather than the start are intentionally not provided. Such selectors would interfere with determining the number of fragments.

Depending on future discussions, this ''::nth-fragment(an+b)'' syntax may be replaced with the new ''::fragment:nth(an+b)'' syntax.

Styling of fragments

Should this apply to fragment overflow only, or also to paginated overflow? (If it applies, then stricter property restrictions would be needed for paginated overflow.)

In the absence of rules with ''::nth-fragment()'' pseudo-elements, the computed style for each fragment box is the computed style for the element for which the fragment box was created. However, the style for a fragment box is also influenced by rules whose selector's subject [[!SELECT]] has an ''::nth-fragment()'' pseudo-element, if the 1-based number of the fragment box matches that ''::nth-fragment()'' pseudo-element and the selector (excluding the ''::nth-fragment()'' pseudo-element) matches the element generating the fragments.

When determining the style of the fragment box, these rules that match the fragment pseudo-element cascade together with the rules that match the element, with the fragment pseudo-element adding the specificity of a pseudo-class to the specificity calculation. Does this need to be specified in the cascading module as well?

<!DOCTYPE HTML>
<style>
  .bouncy-columns {
    overflow: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .bouncy-columns::nth-fragment(1) {
    background: aqua; color: black;
    transform: rotate(-3deg);
  }
  .bouncy-columns::nth-fragment(2) {
    background: yellow; color: black;
    transform: rotate(3deg);
  }
</style>
<div class="bouncy-columns">
  ...
</div>

In this
example, the
text in the div
is broken into
a series of
columns. The
author
probably

intended the
text to fill two
columns. But
if it happens to
fill three
columns, the
third column is
still created. It

just doesn't
have any
fragment-specific
styling because
the author
didn't give it
any.

Styling an ''::nth-fragment()'' pseudo-element with the 'overflow' property does take effect; if a fragment box has a computed value of 'overflow' other than ''fragments'' then that fragment box is the last fragment. However, overriding 'overflow' on the first fragment does not cause the fragment box not to exist; whether there are fragment boxes at all is determined by the computed value of overflow for the element. Need to reword this to refer to the appropriate choice of 'overflow-x' or 'overflow-y', and then point to rule about the handling of the other one of 'overflow-x' or 'overflow-y'.

Styling an ''::nth-fragment()'' pseudo-element with the 'content' property has no effect; the computed value of 'content' for the fragment box remains the same as the computed value of content for the element.

Specifying ''display: none'' for a fragment box causes the fragment box with that index not to be generated. However, in terms of the indices used for matching ''::nth-fragment()'' pseudo-elements of later fragment boxes, it still counts as though it was generated. However, since it is not generated, it does not contain any content.

Specifying other values of 'display', 'position', or 'float' is permitted, but is not allowed to change the computed value of 'display-inside'. (Since 'overflow', 'overflow-x', and 'overflow-y' only apply to block containers, flex containers, and grid containers the computed value of 'display-inside' is always ''display-inside/block'', ''display-inside/flex'', or ''display-inside/grid''. Need to specify exactly how this works, but it depends on having 'display-inside' and 'display-outside' specified.

To match the model for other pseudo-elements where the pseudo-elements live inside their corresponding element, declarations in ''::nth-fragment()'' pseudo-elements override declarations in rules without the pseudo-element. The relative priority within such declarations is determined by normal cascading order (see [[!CSS21]]).

Styles specified on ''::nth-fragment()'' pseudo-elements do affect inheritance to content within the fragment box. In other words, the content within the fragment box must inherit from the fragment box's style (i.e., the pseudo-element style) rather than directly from the element. This means that elements split between fragment boxes may have different styles for different parts of the element.

This inheritance rule allows specifying styles indirectly (by using explicit ''inherit'' or using default inheritance on properties that don't apply to '':first-letter'') that can't be specified directly (based on the rules in the next section). This is a problem. The restrictions that apply to styling inside fragments should also apply to inheritance from fragments.

<!DOCTYPE HTML>
<style>
  .article {
    overflow: fragments;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    margin-bottom: 1em;
    height: 4em;
  }
  .article::nth-fragment(2) {
    margin-left: 5em;
    margin-right: 2em;
  }
</style>
<div class="article">
  The <code>font-size</code> property...
</div>

The font-size property
specified on the fragment
is inherited into the

descendants of the fragment.
This means that inherited
properties can be used
reliably on a fragment, as in
this example.

Styling inside fragments

Should this apply to fragment overflow only, or also to paginated overflow, or even to pagination across pages?

The ''::nth-fragment()'' pseudo-element can also be used to style content inside of a fragment box. Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements, the ''::nth-fragment()'' pseudo-element can be applied to parts of the selector other than the subject: in particular, it can match ancestors of the subject. However, the only CSS properties applied by rules with such selectors are those that apply to the ''::first-letter'' pseudo-element.

To be more precise, when a rule's selector has ''::nth-fragment()'' pseudo-elements attached to parts of the selector other than the subject, the declarations in that rule apply to a fragment (or pseudo-element thereof) when:

<!DOCTYPE HTML>
<style>
  .dark-columns {
    overflow: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin-right: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .dark-columns::nth-fragment(1) {
    background: aqua; color: black;
  }
  .dark-columns::nth-fragment(1) :link {
    color: blue;
  }
  .dark-columns::nth-fragment(1) :visited {
    color: purple;
  }
  .dark-columns::nth-fragment(2) {
    background: navy; color: white;
  }
  .dark-columns::nth-fragment(2) :link {
    color: aqua;
  }
  .dark-columns::nth-fragment(2) :visited {
    color: fuchsia;
  }
</style>
<div class="dark-columns">
  ...
</div>

In this
example, the
text flows
from one
light-colored
fragment into
another
dark-colored

fragment. We
therefore want
different styles
for hyperlinks
in the different
fragments.

The 'max-lines' property

Authors may wish to style the opening lines of an element with different styles by putting those opening lines in a separate fragment. However, since it may be difficult to predict the exact height occupied by those lines in order to restrict the first fragment to that height, this specification introduces a 'max-lines' property that forces a fragment to break after a specified number of lines. This forces a break after the given number of lines contained within the element or its descendants, as long as those lines are in the same block formatting context.

Name:	max-lines
Value:	none \| <integer>
Initial:	none
Applies to:	fragment boxes
Inherited:	no
Animatable:	as integer
Percentages:	N/A
Media:	visual
Computed value:	specified value
Canonical order:	per grammar

<!DOCTYPE HTML>
<style>
  .article {
    overflow: fragments;
  }
  .article::first-letter {
    font-size: 2em;
    line-height: 0.9;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    max-lines: 3;
  }
  .article::nth-fragment(2) {
    column-count: 2;
  }
</style>
<div class="article">
  ...
</div>

The max-lines property allows
authors to use a larger font for the first
few lines of an article. Without the

max-lines property, authors
might have to use the
'height' property instead, but
that would leave a slight gap
if the author miscalculated
how much height a given
number of lines would
occupy (which might be

particularly hard if the author
didn't know what text would
be filling the space, exactly
what font would be used, or
exactly which platform's font
rendering would be used to
display the font).

Overflow in static media

This specification should define useful behavior for all values of 'overflow' in static media (such as print). Current implementation behavior is quite poor and produces unexpected results when authors have not considered what will happen when the content they produce for interactive media is printed.

Acknowledgments

Thanks especially to the feedback from Rossen Atanassov, Bert Bos, Tantek Çelik, John Daggett, fantasai, Daniel Glazman, Vincent Hardy, Håkon Wium Lie, Peter Linss, Robert O'Callahan, Florian Rivoal, Alan Stearns, Steve Zilles, and all the rest of the www-style community.

CSS Overflow Module Level 3

Introduction