[cssom] Don't check for @charset in insertRule now that css-syntax does not produce it. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=27422

Author: zcorpan

cssom/Overview.bs

@@ -1563,11 +1563,11 @@ must be run:
  empty string terminate these steps.
 
  <li>Let <var>input URL</var> be the value specified.
- 
+
  Issue: Be more specific
 
  <li>Let <var>base URL</var> be the document's <a data-anolis-spec=html>document base URL</a>.
- 
+
  Issue: Is there a document at this point?
 
  <li>Let <var>referrer</var> be the document's <a lt=concept-document-url>address</a>.
@@ -1581,7 +1581,7 @@ must be run:
 
  <li>Let <var>response</var> be the result of <a lt="fetch a CSS style sheet">fetching a CSS style sheet</a> with parsed URL <var>parsed URL</var>,
  referrer <var>referrer</var> and document being the document.
- 
+
  Issue: What if the HTML parser hasn't decided on quirks/non-quirks yet?
 
  <li>
@@ -1726,8 +1726,6 @@ To <dfn>insert a CSS rule</dfn> <var>rule</var> in a CSS rule list <var>list</va
  on argument <var>rule</var>.
  <li>If <var>new rule</var> is a syntax error, <a>throw</a>
  a {{SyntaxError}} exception.
- <li>If <var>new rule</var> is an <code>@charset</code> at-rule, <a>throw</a> a
- {{SyntaxError}} exception.
  <li>Set <var>length</var> to the number of items in <var>list</var>.
  <li>If <var>index</var> is greater than <var>length</var>, then <a>throw</a>
  an {{IndexSizeError}} exception.