CSS Region Styling Module Level 3
Unofficial Draft 7 August 2012
- Issue Tracking:
- Maintained in document (only editor's draft is current)
- Editors:
- L. David Baron, Mozilla
CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This module contains the features of CSS relating to new mechanisms of overflow handling in visual media (e.g., screen or paper). In interactive media, it describes features that allow the overflow from a fixed size container to be handled by pagination (displaying one page at a time). It also describes features, applying to all visual media, that allow the contents of an element to be spread across multiple containers, allowing the contents to flow across multiple regions of a page or to have different styles for different parts.
This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as 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-region-styling” in the subject, preferably like this: “[css3-region-styling] …summary of comment…”
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The following features are at risk: …
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 a region 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 region into which the content of an element must flow, this specification allows authors to specify multiple regions, 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].
overflow:paginate or overflow:pages (or paged-x, paged-y, paged-x-controls, paged-y-controls as css3-gcpm has?)
Ability to display N pages at once rather than just one page at once?
This section introduces and defines the meaning of the new ‘regions’ (or ‘repeat’?) value of the ‘overflow’ property.
When the computed value of ‘overflow’
for an element is ‘regions’, and
implementations would otherwise have created a box for the element, then
implementations must create at least one box for that element. Each box
created for the element is called a region box
for that element. (If an element with ‘overflow:
regions’ generates only one box, that box is a region box. However, if an element's
computed ‘overflow’ is not ‘regions’, then its box is not a region box.) Every region box is a fragmentation container, and
for each region box which ends with a
fragmentation break, (which could happen because breakable content
overflows in the block dimension or because of a forced break), there must
be another region 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. (Breakable content might overflow in
the box dimension either because of a specified size on the region box or
because the region box is within a fragmentation context in which it is
being broken. In other words, a single region box is never broken across
columns or pages; the pieces that are in separate columns or pages are
always distinct region boxes.)
We also want ‘::nth-region()’
pseudo-elements to be able to apply to the pieces of an element split
within a fragmentation context. Should we require that authors who want to
use ‘::nth-region()’ in this way specify
‘overflow:regions’ (even if they don't specify
a constrained height), or should it work automatically for all elements
even if they don't have ‘overflow: regions’?
<!DOCTYPE HTML>
<title>Breaking content into
equal-sized cards</title>
<style>
.in-cards {
overflow: regions;
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. |
The ::nth-region() pseudo-element is a pseudo-element that describes some of the region 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 region boxes generated by the element instead of siblings of the element.
If we go with a name for the ‘overflow’ value other than ‘overflow: regions’, then we should probably rename this
pseudo-element, perhaps to ::nth-piece(), ::nth-part(), or
::nth-instance().
Selectors that allow addressing regions by counting from the end rather than the start are intentionally not provided. Such selectors would interfere with determining the number of regions.
Should this apply to region 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-region()’ pseudo-elements, the computed style for
each region box is the computed style for
the element for which the region box was
created. However, the style for a region
box is also influenced by rules whose selector's subject [SELECT] has an
‘::nth-region()’ pseudo-element, if the 1-based
number of the region box matches that ‘::nth-region()’ pseudo-element and the selector
(excluding the ‘::nth-region()’ pseudo-element)
matches the element generating the regions.
<!DOCTYPE HTML>
<style>
.bouncy-columns {
overflow: regions;
width: 6em;
height: 10em;
float: left;
margin: 1em;
font: medium/1.25 Times New
Roman, Times, serif;
}
.bouncy-columns::nth-region(1) {
background: aqua; color: black;
transform: rotate(-3deg);
}
.bouncy-columns::nth-region(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 region-specific styling because the author didn't give it any. |
Styling an ‘::nth-region()’ pseudo-element
with the ‘overflow’ property has no
effect; the computed value of ‘overflow’
for the region box remains the same as the computed value of overflow for
the element.
Specifying ‘display: none’ for a region box
causes the region box with that index not to be generated. However, in
terms of the indices used for matching ‘::nth-region()’ pseudo-elements of later region boxes,
it still counts as though it was generated. However, since it is not
generated, it does not contain any content.
Would it make more sense to forbid ‘display:none’? Or perhaps to forbid ‘display’, ‘position’, ‘float’, and similar (in addition to ‘overflow’)?
To match the model for other pseudo-elements where the pseudo-elements
live inside their corresponding element, declarations in ‘::nth-region()’ 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-region()’
pseudo-elements do affect inheritance to content within the region box. In other words, the content
within the region box must inherit from
the region box's style (i.e., the pseudo-element style) rather than
directly from the element. This means that elements split between region
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 regions should also apply to inheritance from
regions.
<!DOCTYPE HTML>
<style>
.article {
overflow: regions;
}
.article::nth-region(1) {
font-size: 1.5em;
margin-bottom: 1em;
height: 4em;
}
.article::nth-region(n+2) {
/* 2 and up */
margin-left: 5em;
margin-right: 2em;
}
</style>
<div class="article">
The <code>font-size</code> property...
</div>
|
The
font-size
propertyspecified on the region is inherited into the descendants of the
region.
This means that inherited properties can be used reliably on a region, as in this example. |
Should this apply to region overflow only, or also to paginated overflow, or even to pagination across pages?
The ‘::nth-region()’ pseudo-element can also
be used to style content inside of a region
box. Unlike the ‘::first-line’ and
‘::first-letter’ pseudo-elements, the ‘::nth-region()’ 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-region()’ 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:
::first-letter’ pseudo-element,
::nth-region()’
pseudo-elements been removed, with a particular association between each
sequence of simple selectors and the element it matched, and
::nth-region()’
pseudo-element, the fragment lives within a region box of the element associated in
that association with the selector that the pseudo-element was attached
to, and whose index matches the pseudo-element.
<!DOCTYPE HTML>
<style>
.dark-columns {
overflow: regions;
width: 6em;
height: 10em;
float: left;
margin-right: 1em;
font: medium/1.25 Times New
Roman, Times, serif;
}
.dark-columns::nth-region(1) {
background: aqua; color: black;
}
.dark-columns::nth-region(1) :link {
color: blue;
}
.dark-columns::nth-region(1) :visited {
color: purple;
}
.dark-columns::nth-region(2) {
background: navy; color: white;
}
.dark-columns::nth-region(2) :link {
color: aqua;
}
.dark-columns::nth-region(2) :visited {
color: fuchsia;
}
</style>
<div class="dark-columns">
...
</div>
|
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 region.
However, since it may be difficult to predict the exact height occupied by
those lines in order to restrict the first region to that height, this
specification introduces a ‘max-lines’ property that forces a region 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: | region boxes |
| Inherited: | no |
| Percentages: | N/A |
| Media: | visual |
| Computed value: | specified value |
| Animatable: | as integer |
| Canonical order: | per grammar |
Breaks occur only as specified elsewhere.
In addition to any breaks specified elsewhere, a break is forced before any line that would exceed the given number of lines being placed inside the element (excluding lines that are in a different block formatting context from the block formatting context to which an unstyled child of the element would belong).
If there are multiple boundaries between this line and the previous, where exactly (in terms of element boundaries) is the break forced?
Only positive integers are accepted. Zero or negative integers are a parse error.
Should this apply to regions overflow only, or also to pagination?
<!DOCTYPE HTML>
<style>
.article {
overflow: regions;
}
.article::first-letter {
font-size: 2em;
line-height: 0.9;
}
.article::nth-region(1) {
font-size: 1.5em;
max-lines: 3;
}
.article::nth-region(n+2) {
/* 2 and up */
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, butthat 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). |
This module extends the ‘overflow’
feature defined in [CSS21] section 11.1.1. It defines
additional overflow handling mechanisms that implementations must
implement as described in this module in order to conform to this module.
No properties in this module apply to the ::first-line or
::first-letter pseudo-elements.
This specification follows the CSS property definition conventions from [CSS21]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
class="example", like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from
the normative text with class="note", like this:
Note, this is an informative note.
Conformance to CSS Region Styling Module Level 3 is defined for three conformance classes:
A style sheet is conformant to CSS Region Styling Module Level 3 if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.
A renderer is conformant to CSS Region Styling Module Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Region Styling Module Level 3 by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)
An authoring tool is conformant to CSS Region Styling Module Level 3 if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.
Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]
For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
Thanks especially to the feedback from Håkon Wium Lie, Florian Rivoal, Alan Stearns, and all the rest of the www-style community.
| Property | Values | Initial | Applies to | Inh. | Percentages | Media |
|---|---|---|---|---|---|---|
| max-lines | none | <integer> | none | region boxes | no | N/A | visual |