[css-syntax-3] Define at-rule and descriptor. w3c#5633

Author: fantasai

css-syntax-3/Overview.bs

@@ -3361,6 +3361,40 @@ Style rules</h3>
 		For example, qualified rules inside ''@media'' rules [[CSS3-CONDITIONAL]] are style rules,
 		but qualified rules inside ''@keyframes'' rules are not [[CSS3-ANIMATIONS]].
 
+<h3 id="at-rules">
+At-rules</h3>
+
+	An <dfn export>at-rule</dfn> is a rule that begins with an at-keyword,
+	and can thus be distinguished from [=style rules=] in the same context.
+
+	[=At-rules=] are used to:
+
+	* group and structure style rules and other at-rules
+		such as in [=conditional rules=]
+	* declare style information that is not associated with a particular element,
+		such as defining [=counter styles=]
+	* manage syntactic constructs
+		such as [=imports=] and [=namespace=] keyword mappings
+	* and serve other miscellaneous purposes not served by a [=style rule=]
+
+	At-rules take many forms, depending on the specific rule and its purpose,
+	but broadly speaking there are two kinds:
+	<dfn lt="statement at-rule" export>statement at-rules</dfn>
+	which are simpler constructs that end in a semicolon,
+	and <dfn lt="block at-rule" export>block at-rules</dfn>
+	which end in a [={}-block=]
+	that can contain nested [=rules=] or [=declarations=].
+
+	[=Block at-rules=] will typically contain a collection of
+	(generic or [=at-rule=]&ndash;specific)
+	[=at-rules=], [=qualified rules=], and/or [=descriptor declarations=]
+	subject to limitations defined by the [=at-rule=].
+	<dfn lt="descriptor" export for=CSS>Descriptors</dfn> are similar to [=properties=]
+	(and are declared with the same syntax)
+	but are associated with a particular type of [=at-rule=]
+	rather than with elements and boxes in the tree.
+
+
 <h3 id='charset-rule'>
 The ''@charset'' Rule</h3>