The specification describes a CSS box model optimized for layout isolation that can guarantee the layout of the descendants of an element do not impact the layout of the element itself nor its ancestors and siblings, like an embedded document does, which enables new optimizations.
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This section is not normative.
CSS Layouts have always been exhaustive in the sense the layout of most elements can potentially depend on the layout of every other element in the page, including its own children.
This characteristic has an important impact on the layout efficiency: when the DOM tree is modified, the layout of every element of the page may be impacted by the change. In the case of infinitely scrolling list nested in complex web pages, the impact of adding elements in the DOM tree during a scroll event can be highly visible.
The second issue is that it’s impossible to make the layout of some elements depend on the size of a parent element. A very concrete use case for this is user components: the layout of a list view may change from a 2D grid to a 1D list depending on the size it can occupy. This very simple thing is not possible do so using the current CSS layouts because constraints solving would be slow and inefficient.
The third issue is that, when a wrapper or a web component is moved on a web page (expect in some particular and optimized conditions, which depends from browser to browser), the layout/painting of the whole page needs to be recomputed, including the layout of the elements inside the moved element, while their layout should not depend on the position of the wrapper on the page.
To solve those problems, this specification introduces the notion of
This module defines a new value for the '
property defined in CSS 2.1.
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
when combined with this module, expands the definition of the
<color> value type as used in this specification.
To solve both problems, a solution already exists: using an
<iframe> to host the user component, let it be a scrolling list or
anything else (by <iframe> I mean a fixed-size, non-flattened
and non-seamless iframe).
Using an iframe solve both problems. Firstly, the layout inside the <iframe> is independent of the layout outside the iframe. When an iframe is moved, if its dimension doesn’t change, the layout of the document inside the iframe doesn’t have to be recomputed. Also, even when the dimension of the iframe changes, the layout of the document inside the iframe can be delayed and doesn’t impact the layout of the outer document. Last but not least, if the DOM tree inside the frame changes, it doesn't force a relayout of the outer document.
Naturally, the limitation imposed by this setting (in terms of layout) is that the size of the iframe must not depend on the layout of its content (even if seamless iframes have already lifted some of those restrictions, and even if scripts from the inner document have the potential to retroact on the parent iframe element, something that was already done in the past to polyfill seamless iframes).
A local viewport host is a leaf node of the layout tree from which a new layout tree is spawned (whose root is called the local viewport root and whose children are the children that would have been put in the local viewport host otherwhise). That shadow layout tree is independent from the one it has been spawned, even if they share the same browsing context.
To the contrary of the case of an iframe, the new context applies to
CSS and to the layout engine only, and not to the real DOM; it doesn't
create a nested browsing context either. From a scripting point of view,
there's no visible difference between an element inside or outside the
It is possible to combine a local viewport with a Shadow DOM to get both Layout and DOM isolation. It's suspected that a lot of web components will actually use this to improve their layout performance (and be isolated from the document in which they are inserted).
Local viewports can take advantage of
multi-cores processors by running the layout of independent viewports on
different threads: on the following example, the root element is divided
into 4 viewports (à la frameset) which make it possible to do
the layout of the page on 4 independent threads.
If only one section of the page is not responding fast enough for a
60fps frame rate, it's still possible to reuse the last calculated
rendering for that section and still get 60fps rendering for the rest of
Inside a local viewport, a new layout context is applied as if the element in the spawned layout tree were hosted in a different viewport whose dimensions match the ones of the local viewport host.
That means that, for example, the value of CSS counters are not shared
between a viewport and its local viewports. It's possible, however, to
maintain the relationship by script using counter-reset, if this is
The parent viewport of an element is the viewport host of the layout tree in which an element is rendered. If there's no such element, the parent viewport of the element is the viewport defined in the section 9.1 of the CSS 2.1 specification.
As long as the parent viewport of an element is not resized and its
content & styles are not modified, the layout of an element should not
An element or a pseudo-element is represented with a local viewport host
in the layout tree if its '
display' property has
The local viewport host on the main document is rendered as usual
in its viewport context, except that it doesn't have any child. Whenever
the local viewport host should be rendered, the render of the local
viewport root is painted instead of its actual content.
The local viewport root inherits the computed value of its
overflow property from the local viewport host it's
attached to. The children of the local viewport root are rendered as if
they were direct child of the root element of the viewport.
When rendered, the content of the viewport is clipped to the dimensions
of the parent local viewport host border box. This restriction is
necessary to make sure that the painted area of a viewport host is not
dependent of its content.
What to do if 'height: auto' is specified on a local viewport host? If
we follow the current algorithm, auto will result in an
height (because the element is seen as having no children from the
parent viewport). An option would be to resolve this by using the same
algorithm as a seamless iframe does in HTML5.
How to specify the layout manager in application inside a viewport
We could-use a
viewport-display property for
the purpose, of transform modify the grammar to accept (
<display-type>? <display-inside> |
can be either '
viewport' or '
(the latter being the default value).
Tab Atkins also proposed a new layout manager where the viewports can
only have very specific, optimized layout modes but I don't think this
is a good idea to force that as the only possibility (the mean use case
of a viewport is really to isolate the layout of a part of the DOM tree,
not to force a particular layout mode)
This section is empty and is a work in progress.
I would also like, for scripting purpose, to get the ‘resize’ event working on the viewport elements (in fact, I would like it to work on any element, but at least on viewport elements this is critical). This event is important for case of web components that should be able to perform actions when their size changes.
It’s interesting to note that the ‘resize’ event already work on all
elements (firing when their width/height changes) on Internet Explorer
since version 5.5 so this is reasonable to think it’s possible to
implement it (even if it has been deprecated in IE10 because the event
don’t work totally correctly in the case of CSS transitions and
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
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 Local Viewports is defined for three conformance classes:
A style sheet is conformant to CSS Local Viewports if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.
A renderer is conformant to CSS Local Viewports if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Local Viewports 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 Local Viewports Module 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 firstname.lastname@example.org mailing list.
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 to Tab Atkins and Boris Zbarsky for their help.
Thanks also to all the persons who contributed to this draft via
www-style or any other mean.
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