[cssom] Throw for insertRule('@charset ...',0). https://www.w3.org/Bugs/Public/show_bug.cgi?id=22207

Author: Simon Pieters

cssom/Overview.html

@@ -995,6 +995,8 @@ <h4 id="the-cssstylesheet-interface"><span class="secno">6.1.2 </span>The <code
  <li><p>Set <var>newRule</var> to the results of performing <a href="#parse-a-css-rule">parse a CSS rule</a>
  on argument <var>rule</var>, <a class="external" href="http://dom.spec.whatwg.org/#concept-throw" title="concept-throw">throwing</a>
  a <code class="external"><a href="http://dom.spec.whatwg.org/#syntaxerror">SyntaxError</a></code> exception if a parse error occurs.</li>
+ <li><p>If <var title="">newRule</var> is an <code title="">@charset</code> at-rule, <a class="external" href="http://dom.spec.whatwg.org/#concept-throw" title="concept-throw">throw</a> a
+ <code class="external"><a href="http://dom.spec.whatwg.org/#syntaxerror">SyntaxError</a></code> exception.
  <li><p>Set <var>N</var> to <code>cssLength.length</code>.</li>
  <li><p>If argument <var>index</var> is greater than <var>N</var>, then <a class="external" href="http://dom.spec.whatwg.org/#concept-throw" title="concept-throw">throw</a>
  an <code class="external"><a href="http://dom.spec.whatwg.org/#indexsizeerror">IndexSizeError</a></code> exception.</li>

cssom/Overview.src.html

@@ -925,6 +925,8 @@ <h4>The <code title>CSSStyleSheet</code> Interface</h4>
  <li><p>Set <var>newRule</var> to the results of performing <span>parse a CSS rule</span>
  on argument <var>rule</var>, <span data-anolis-spec='dom' title='concept-throw'>throwing</span>
  a <code data-anolis-spec='dom'>SyntaxError</code> exception if a parse error occurs.</p></li>
+ <li><p>If <var title>newRule</var> is an <code title>@charset</code> at-rule, <span data-anolis-spec='dom' title='concept-throw'>throw</span> a
+ <code data-anolis-spec='dom'>SyntaxError</code> exception.
  <li><p>Set <var>N</var> to <code>cssLength.length</code>.</p></li>
  <li><p>If argument <var>index</var> is greater than <var>N</var>, then <span data-anolis-spec='dom' title='concept-throw'>throw</span>
  an <code data-anolis-spec='dom'>IndexSizeError</code> exception.</p></li>