This specification defines a pseudo-class designed for the purpose of matching the context element of a query, where the the scope of elements that match a given Selector is constrained to be within a subtree rooted by a particular node.
This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. It probably contains errors. Its publication here does not imply endorsement of its contents by W3C.
:context Pseudo-Class
This section is non-normative.
This specification introduces a special contextual pseudo-class that matches the root element of a subtree that constrains the scope of a given selector to itself and its descendants. This is designed for use with applications of Selectors with features that provide a scoping mechanism, such as Selectors API [SELECTORS-API] and scoped stylesheets within HTML 5 [HTML5].
This section is non-normative.
This selector is required to address the following use cases.
:bound-element, but
it is not the same as it because they will differ when used outside of a
binding. There doesn’t appear to be any way to avoid the redundancy in
this case.
All diagrams, examples and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.
The key words must, should, and may in the normative parts of this document are to be interpreted as described in RFC 2119 [RFC2119].
The following conformance classes are defined (and considered) by this specification:
:context described in
this specification and conforms to all must-level
criteria that apply to implementations.
Element node
within the context in which a selector is being evaluated.
By default, when a given element is being evaluated against a given selector, the context element is, unless otherwise specified in another specification, the root element of the document to which the element belongs.
For example, in Selectors API [SELECTORS-API], when the
querySelector method is invoked on an element, the context element is defined to be the element itself.
In addition to the terms defined in this specification, the terms defined in Selectors [SELECT] and DOM Level 3 Core [DOM-LEVEL-3-CORE] are also used.
Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent.
:context Pseudo-ClassThe :context pseudo-class must match the
context element.
Specifications intending for this pseudo-class to match a specific element other than the document's root element must define which element is the context element.
This may be renamed to :scope.
This section is non-normative
This pseudo-class can be used within a scoped stylesheet in HTML 5 [HTML5] to select the
style element’s parent node. The following example
demonstrates how to apply styles to the context
element and to one of it’s child nodes, without inadvertently
selecting and styling other descendants. The outer article
element will be styled with the border, padding and margin, but the inner
article element containing the reader’s comment will not
be.
This will require HTML5 to specify that the context element is the parent element of a scoped stylesheet.
<section>
<style scoped>
:context { background: #FFC; margin: 1em; padding: 1em; }
:context>article { border-left: 2px solid silver; padding: .5em; margin: 1em 0; }
</style>
<article>
<h1>An Example</h1>
<p>This is an example article.
<h2>Comments</h2>
<article>
<p>First Post!</p>
<footer><em>Comment by A. Troll</em></footer>
</article>
</article>
</section>
The pseudo-class can also be used within queries performed using the methods defined in Selectors API [SELECTORS-API]. The following examples will make use of this sample document fragment.
<div>
<section id="container">
<div>
<p>This is a <em>sample</em> document fragment and this is column 1.
<div>
<p>It contains a couple of paragraphs and a few levels of nested divs.
</div>
</div>
<div>
<p>This is column 2.
</div>
</section>
</div>
Ordinarily, a selector in a query is evaluated against an element in the context of the entire document. This means that if you wish to select descendant elements using the descendant combinator, ancestor elements of the context element will be considered during selector evaulation.
var container = document.getElementById("container");
var div = container.querySelector("div div");
That query will return the first child div element within the container div. This is because the container’s ancestor is also a div, which is matched during selector evaluation. To ensure that only descendant elements are considered, you can use the :context pseudo-class to restrict the scope.
var div = container.querySelector(":context div div");
That one will match the innermost div in the example.
The :context pseudo-class can also be used in conjunction
with the child combinator to ensure that only child elements of the context element are selected.
var columns = container.querySelector(":context>div");
The editors would like to thank to the following people who have contributed to this specification (ordered on first name):