[selectors] Fix our handling of universal selector to not be implied; tweak default namespace definition accordingly, and generally rewirite the namespace/type selector sections.

--HG--
extra : rebase_source : 82ffd366fb3423bab0b50e898199ac14a4936072

Author: tabatkins

selectors/Overview.bs

@@ -657,11 +657,11 @@ Data Model</h3>
 		but not only elements with <a>shadow trees</a> have ''::shadow'' pseudo-elements.
 	</div>
 
-	Four aspects of a DOM element are especially relevant for selectors:
+	Five aspects of a DOM element are especially relevant for selectors:
 
 	<ul>
-		<li>The element's type (also known as tagname),
-			which is a pair of strings: a namespace and a local name.
+		<li>The element's type (also known as tagname), which is a string.
+		<li>The element's namespace, which is a string.
 		<li>An ID, which is a string.
 		<li>Classes, which are strings.
 		<li>Attributes, which are pairs of strings consisting of an attribute name and an attribute value.
@@ -673,12 +673,12 @@ Data Model</h3>
 	are called <dfn export title="feature selector">feature selectors</dfn>.
 
 	While elements may lack any of the individual <a>features</a>,
-	some elements are <dfn export>featureless</dfn>,
-	which means that not only do they lack any <a>features</a>
-	(and thus will never match a <a>feature selector</a>),
-	but any selectors that reference <a>features</a>,
-	such as '':not(div)'',
-	also never match them.
+	some elements are <dfn export>featureless</dfn>.
+	A <a>featureless</a> element does not match any <a>feature selector</a>,
+	or any selector that resolves based on <a>features</a>
+	(such as '':not(div)''),
+	with the exception of namespace selectors--
+	a <a>featureless</a> element is treated as having <em>all</em> namespaces.
 	Thus, only <a>pseudo-classes</a> or <a>pseudo-elements</a> can be used to select them.
 	Individual <a>featureless</a> elements may define additional restrictions on what kinds of selectors can match them.
 
@@ -687,6 +687,25 @@ Data Model</h3>
 		and can't be matched by <em>any</em> <a>pseudo-class</a> except for '':host'' and '':host-context()''.)
 	</div>
 
+	<details class='why'>
+		<summary>Why do featureless elements match all namespaces?</summary>
+
+		In a previous version of Selectors,
+		all <a>compound selectors</a> contained a <a>type selector</a>;
+		if you didn't supply one explicitly,
+		a <a>universal selector</a> was implied.
+		Default namespaces [[CSS3NAMESPACE]] affected the implied universal selector,
+		so that selectors like '':hover'' actually meant ''defaultNS|*:hover''.
+
+		The universal selector is no longer implied,
+		but default namespaces still exist,
+		and have to apply to all compound selectors.
+		The behavior of <a>featureless</a> elements,
+		like the <a>host element</a> in a shadow tree,
+		thus needs to accommodate that and work appropriately.
+		In this case, "appropriately" means "ignoring the default namespace".
+	</details>
+
 	When matching a selector against a document which is in <a>quirks mode</a>,
 	class and ID selectors must be matched <a title="ASCII case-insensitive">ASCII case-insensitively</a>
 	against the classes and ID of the elements in the document.
@@ -724,9 +743,9 @@ Guidance on Mapping Document Languages to DOM</h4>
 
 			If this "type" can be separated into a "basic" name
 			and a "namespace" that groups names into higher-level groups,
-			that should be reflected in the "namespace" and "local name" divisions of the "type" <a>feature</a>.
-			Otherwise, the "namespace" part should be the empty string,
-			and the entire name should be in the "local name" part.
+			the latter should be reflected as the "namespace" <a>feature</a>.
+			Otherwise, the element shouldn't have a "namespace" <a>feature</a>,
+			and the entire name should be reflected as the "type" <a>feature</a>.
 
 		<dt>id
 		<dd>
@@ -1203,8 +1222,11 @@ The Matches-Any Pseudo-class: '':matches()''</h3>
 	Pseudo-elements cannot be represented by the matches-any pseudo-class;
 	they are not valid within '':matches()''.
 
-	Default namespace declarations do not affect any “implied” universal selectors
-	within a '':matches()'' pseudo-class.
+	Within a '':matches()'' pseudo-class,
+	default namespace declarations only have an effect on compound selectors
+	that contain a <a>type selector</a>.
+
+	Issue: Do we want to extend this protection from default namespaces to other things?
 
 	<div class="example">
 		For example, the following selector matches any element that is being
@@ -1301,51 +1323,62 @@ Elemental selectors</h2>
 Type (tag name) selector</h3>
 
 	A <dfn export>type selector</dfn> is the name of a document language element type
-	written using the syntax of <a href="http://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified names</a> [[!CSS3NAMESPACE]].
+	written as either an <a>identifier</a>
+	or, if interacting with namespaces,
+	a <a>CSS qualified name</a>
+	(see [[#type-nmsp]])
+	[[!CSS3NAMESPACE]].
+
 	A type selector represents an instance of the element type in the document tree.
 
 	<div class="example">
-	 Example:
-	 The following selector represents an <a element>h1</a> element in the document tree:
-
-	 <pre>h1 </pre>
+		For example, the selector ''h1'' represents an <a element>h1</a> element in the document.
 	</div>
 
-<h4 id="typenmsp">
-Type selectors and namespaces</h4>
+<h3 id="the-universal-selector">
+Universal selector </h3>
 
-	Type selectors allow an optional namespace component: a namespace
-	prefix that has been previously <a href="#nsdecl">declared</a> may be
-	prepended to the element name separated by the namespace separator
-	&quot;vertical bar&quot; (U+007C, <code>|</code>). (See, e.g., [[XML-NAMES]] for the use of namespaces in
-	XML.)
+	The <dfn export>universal selector</dfn> is a special <a>type selector</a>,
+	written either as the asterisk character (<code>*</code> U+002A),
+	or as a <a>CSS qualified name</a>
+	with an asterisk as the local name (see [[#type-nmsp]]).
+	It represents an element with any name.
 
-	The namespace component may be left empty (no prefix before the
-	namespace separator) to indicate that the selector is only to
-	represent elements with no namespace.
+	Like a <a>type selector</a>,
+	the <a>universal selector</a> can be qualified by a namespace,
+	and is affected by a default namespace as defined in [[#type-nmsp]].
 
-	An asterisk may be used for the namespace prefix, indicating that
-	the selector represents elements in any namespace (including elements
-	with no namespace).
+	Unless an element is <a>featureless</a>,
+	the presence of a <a>universal selector</a> has no effect on whether the element matches the selector.
+	(<a>Featureless</a> elements do not match any <a>feature</a> selector,
+		including the <a>universal selector</a>.)
 
-	Element type selectors that have no namespace component (no
-	namespace separator) represent elements without regard to the
-	element's namespace (equivalent to "<code>*|</code>") unless a default
-	namespace has been <a href="#nsdecl">declared</a>  for namespaced selectors (e.g. in CSS, in
-	the style sheet). If a default namespace has been declared, such
-	selectors will represent only elements in the default namespace.
+	<div class="example">
+		<ul>
+			<li>
+				<span class=css>*[hreflang|=en]</span> and <span class=css>[hreflang|=en]</span> are equivalent,
+			<li>''*.warning'' and ''.warning'' are equivalent,
+			<li>''*#myid'' and ''#myid'' are equivalent.
+		</ul>
+	</div>
 
-	A type selector containing a namespace prefix that has not been
-	previously <a href="#nsdecl">declared</a> for namespaced selectors is
-	an <a href="#conformance">invalid</a> selector.
+	The <a>universal selector</a> follows the same syntax rules as other <a>type selectors</a>:
+	only one can appear per <a>compound selector</a>,
+	and it must be the first <a>simple selector</a> in the <a>compound selector</a>.
 
-	In a namespace-aware client,
-	the name part of element type selectors
-	(the part after the namespace separator, if it is present)
-	will only match against the <a href="http://www.w3.org/TR/REC-xml-names/#NT-LocalPart">local part</a>
-	of the element's <a href="http://www.w3.org/TR/REC-xml-names/#ns-qualnames">qualified name</a>.
+	Note: In some cases, adding a <a>universal selector</a> can make a selector easier to read,
+	even though it has no effect on the matching behavior.
+	For example, ''div :first-child'' and ''div:first-child'' are somewhat difficult to tell apart at a quick glance,
+	but writing the former as ''div *:first-child'' makes the difference obvious.
 
-	In summary:
+<h3 id='type-nmsp'>
+Namespaces in Selectors</h3>
+
+	<a>Type selectors</a> allow an optional namespace component:
+	a namespace prefix that has been previously <a href="#nsdecl">declared</a>
+	may be prepended to the element name separated by the namespace separator “vertical bar” (<code>|</code> U+007C).
+	(See, e.g., [[XML-NAMES]] for the use of namespaces in XML.)
+	It has the following meaning in each form:
 
 	<dl>
 		<dt><code>ns|E</code>
@@ -1398,70 +1431,23 @@ Type selectors and namespaces</h4>
 		namespace has been defined.
 	</div>
 
-<h3 id="the-universal-selector">
-Universal selector </h3>
-
-	The <dfn export>universal selector</dfn>, written as a
-	<a href="http://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
-	name</a> [[!CSS3NAMESPACE]] with an asterisk (<code>*</code>
-	U+002A) as the local name, represents the qualified name of any element
-	type. It represents any single element in the document tree in any
-	namespace (including those without a namespace) if no default
-	namespace has been specified for selectors. If a default namespace has
-	been specified, see <a href="#univnmsp">Universal selector and
-	Namespaces</a> below.
-
-	If a universal selector represented by ''*''
-	(i.e. without a namespace prefix)
-	is not the only component of a <a>compound selector</a>
-	or is immediately followed by a <a>pseudo-element</a>,
-	then the ''*'' may be omitted
-	and the universal selector's presence implied.
-
-	<div class="example">
-		<ul>
-			<li>''*[hreflang|=en]'' and ''[hreflang|=en]'' are equivalent,
-			<li>''*.warning'' and ''.warning'' are equivalent,
-			<li>''*#myid'' and ''#myid'' are equivalent.
-		</ul>
-	</div>
-
-	Note: It is recommended that the ''*'' not be omitted,
-	because it decreases the potential confusion between,
-	for example, ''div :first-child''
-	and ''div:first-child''.
-	Here, ''div *:first-child'' is more readable.
-
-<h4 id="univnmsp">
-Universal selector and namespaces</h4>
-
-	The universal selector allows an optional namespace component. It
-	is used as follows:
-
-	<dl>
-		<dt>''ns|*''
-		<dd>
-			all elements in namespace ns
+	If a <a>default namespace</a> is declared,
+	<a>compound selectors</a> without <a>type selectors</a> in them
+	still only match elements in that default namespace.
 
-		<dt>''*|*''
-		<dd>
-			all elements
+	<div class='example'>
+		For example,
+		in the following stylesheet:
 
-		<dt>''|*''
-		<dd>
-			all elements without a namespace
+		<pre>
+		@namespace url("http://example.com/foo");
 
-		<dt>''*''
-		<dd>
-			if no default namespace has been specified,
-			this is equivalent to *|*.
-			Otherwise it is equivalent to ns|*
-			where ns is the default namespace.
-	</dl>
+		.special { ... }
+		</pre>
 
-	A universal selector containing a namespace prefix
-	that has not been previously <a href="#nsdecl">declared</a>
-	is an <a>invalid selector</a>.
+		The ''.special'' selector only matches elements in the "http://example.com/foo" namespace,
+		even though no reference to the type name (which is paired with the namespace in the DOM) appeared.
+	</div>
 
 
 <h2 id="attribute-selectors">