[cssom] Define the serialization of CSSKeyframesRule and CSSKeyframesRule

Part of w3c#4828.

Author: emilio

cssom-1/Overview.bs

@@ -1561,7 +1561,7 @@ To <dfn export>serialize a CSS rule</dfn>, perform one of the following in accor
    <li>The string "<code>@media</code>", followed by a single SPACE (U+0020).</li>
    <li>The result of performing <a>serialize a media query list</a> on rule's media query list.</li>
    <li>A single SPACE (U+0020), followed by the string "{", i.e., LEFT CURLY BRACKET (U+007B), followed by a newline.</li>
-   <li>The result of performing <a>serialize a CSS rule</a> on each rule, separated by a newline and indented by two spaces, in rule's CSSRules list.</li>
+   <li>The result of performing <a>serialize a CSS rule</a> on each rule in the rule's {{CSSGroupingRule/cssRules}} list, separated by a newline and indented by two spaces.</li>
    <li>A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)</li>
   </ol>
 
@@ -1636,8 +1636,32 @@ To <dfn export>serialize a CSS rule</dfn>, perform one of the following in accor
  <a lt="serialize a URL">serialization as URL</a> of the
  {{CSSNamespaceRule/namespaceURI}}
  attribute, followed the character "<code>;</code>" (U+003B).
+
+ <dt>{{CSSKeyframesRule}}
+ <dd>
+  The result of concatenating the following:
+  <ol>
+   <li>The literal string "<code>@keyframes</code>", followed by a single SPACE (U+0020).
+   <li>The <a lt="serialize an identifier">serialization as an identifier</a> of the
+ {{CSSKeyframesRule/name}} attribute.
+   <li>The result of performing <a>serialize a CSS rule</a> on each rule in the rule's {{CSSKeyframesRule/cssRules}} list, separated by a newline and indented by two spaces.</li>
+   <li>A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)</li>
+  </ol>
+
+ <dt>{{CSSKeyframeRule}}
+ <dd>
+  The result of concatenating the following:
+  <ol>
+   <li>The {{CSSKeyframeRule/keyText}}.
+   <li>The string "<code> { </code>", i.e., a single SPACE (U+0020), followed by LEFT CURLY BRACKET (U+007B),
+   <li>The result of performing <a>serialize a CSS declaration block</a> on the rule's associated declarations.
+   <li>If the rule is associated with one or more declarations, the string "<code> </code>", i.e., a single SPACE (U+0020).
+   <li>The string "<code>}</code>", RIGHT CURLY BRACKET (U+007D).
+  </ol>
 </dl>
 
+Issue: The "indented by two spaces" bit matches browsers, but needs work, see <a href="https://github.com/w3c/csswg-drafts/issues/5494">#5494</a>
+
 To <dfn export>insert a CSS rule</dfn> <var>rule</var> in a CSS rule list <var>list</var> at index <var>index</var>, follow these steps:
 <ol>
  <li>Set <var>length</var> to the number of items in <var>list</var>.