[cssom-1] Throw explicit SyntaxError DOMException (#9272)

cssom-1/Overview.bs

@@ -1032,7 +1032,7 @@ The <dfn method for=CSSStyleSheet>insertRule(<var>rule</var>, <var>index</var>)<
  <a>throw</a> a {{SecurityError}} exception.
  <li>If the <a>disallow modification flag</a> is set, throw a {{NotAllowedError}} {{DOMException}}.
  <li>Let <var>parsed rule</var> be the return value of invoking <a>parse a rule</a> with <var>rule</var>.
- <li>If <var>parsed rule</var> is a syntax error, return <var>parsed rule</var>.
+ <li>If <var>parsed rule</var> is a syntax error, throw a {{SyntaxError}} {{DOMException}}.
  <li>If <var>parsed rule</var> is an <a>@import</a> rule, and the <a>constructed flag</a> is set, throw
  a {{SyntaxError}} {{DOMException}}.
  <li>Return the result of invoking <a>insert a CSS rule</a> <var>rule</var> in the <a for=CSSStyleSheet>CSS rules</a>