[css-nesting-1] Add serialization rules, copying from CSSOM.

Author: tabatkins

css-nesting-1/Overview.bs

@@ -747,4 +747,33 @@ invoking [=insert a CSS rule=] <var>rule</var>
 into the [=CSSRule/child CSS rules=] at <var>index</var>.
 
 The <dfn method for=CSSNestingRule>deleteRule(<var>index</var>)</dfn> method
-must [=remove a CSS rule=] from the [=CSSRule/child CSS rules=] at <var>index</var>.
+must [=remove a CSS rule=] from the [=CSSRule/child CSS rules=] at <var>index</var>.
+
+To serialize a {{CSSNestingRule}}:
+return the result of the following steps:
+
+<ol>
+	<li>Let |s| initially be the string "<code>@nest</code>" followed by a single SPACE (U+0020).
+	<li>Append to |s| the result of performing <a>serialize a group of selectors</a> on the rule's associated selectors,
+		followed by the string "<code> {</code>", i.e., a single SPACE (U+0020), followed by LEFT CURLY BRACKET (U+007B).
+	<li>Let |decls| be the result of performing <a>serialize a CSS declaration block</a> on the rule's associated declarations, or null if there are no such declarations.
+	<li>Let |rules| be the result of performing [=serialize a CSS rule=] on each rule in the rule's {{CSSStyleRule/cssRules}} list, or null if there are no such rules.
+	<li>If |decls| and |rules| are both null, append " }" to |s| (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)) and return |s|.
+	<li>If |rules| is null:
+		<ol>
+			<li>Append a single SPACE (U+0020) to |s|
+			<li>Append |decls| to |s|
+			<li>Append " }" to |s| (i.e. a single SPACE (U+0020) followed by RIGHT CURLY BRACKET (U+007D)).
+			<li>Return |s|.
+		</ol>
+	<li>Otherwise:
+		<ol>
+			<li>If |decls| is not null, prepend it to |rules|.
+			<li>For each |rule| in |rules|:
+				<ol>
+					<li>Append a newline followed by two spaces to |s|.
+					<li>Append |rule| to |s|.
+				</ol>
+			<li>Append a newline followed by RIGHT CURLY BRACKET (U+007D) to |s|.
+			<li>Return |s|.
+</ol>