[css-syntax] Add additional example about @font-face.

Author: tabatkins

css-syntax/Overview.html

@@ -4244,15 +4244,21 @@ <h3 id=declaration-rule-list><span class=secno>7.1. </span> Defining Block
    <code>!important</code> is valid and has its usual effect on the cascade
    origin of the property.
 
+  <div class=example> For example, the grammar for ‘<code
+   class=css>@font-face</code>’ in the previous example must, in addition
+   to what is written there, define that the allowed declarations are the
+   descriptors defined in the Fonts spec.</div>
+
   <p> For rules that use <a
    href="#ltrule-list"><var>&lt;rule-list></var></a>, the spec for the rule
    must define what types of rules are valid inside the rule, same as <a
    href="#ltdeclaration-list"><var>&lt;declaration-list></var></a>, and
    unrecognized rules must similarly be removed from the rule's value.
 
-  <div class=example> For example, the grammar in the previous example must,
-   in addition to what is written there, define that the only allowed rules
-   are <var>&lt;keyframe-rule></var>s, which are defined as:
+  <div class=example> For example, the grammar for ‘<code
+   class=css>@keyframes</code>’ in the previous example must, in addition
+   to what is written there, define that the only allowed rules are
+   <var>&lt;keyframe-rule></var>s, which are defined as:
    <pre>&lt;keyframe-rule> = &lt;keyframe-selector> { &lt;declaration-list> }</pre>
 
    <p> Keyframe rules, then, must further define that they accept as

css-syntax/Overview.src.html

@@ -3114,14 +3114,20 @@ <h3 id='declaration-rule-list'>
 		properties containing <code>!important</code> are automatically invalid;
 		otherwise using <code>!important</code> is valid and has its usual effect on the cascade origin of the property.
 
+	<div class='example'>
+		For example, the grammar for ''@font-face'' in the previous example must,
+		in addition to what is written there,
+		define that the allowed declarations are the descriptors defined in the Fonts spec.
+	</div>
+
 	<p>
 		For rules that use <var>&lt;rule-list></var>,
 		the spec for the rule must define what types of rules are valid inside the rule,
 		same as <var>&lt;declaration-list></var>,
 		and unrecognized rules must similarly be removed from the rule's value.
 
 	<div class='example'>
-		For example, the grammar in the previous example must,
+		For example, the grammar for ''@keyframes'' in the previous example must,
 		in addition to what is written there,
 		define that the only allowed rules are <var>&lt;keyframe-rule></var>s,
 		which are defined as: