[cssom] Check index for rule insertion first

Fixes #753.

Author: upsuper

cssom/Overview.bs

@@ -1478,13 +1478,13 @@ To <dfn>serialize a CSS rule</dfn>, perform one of the following in accordance w
 
 To <dfn>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>.
+ <li>If <var>index</var> is greater than <var>length</var>, then <a>throw</a>
+ an {{IndexSizeError}} exception.
  <li>Set <var>new rule</var> to the results of performing <a>parse a CSS rule</a>
  on argument <var>rule</var>.
  <li>If <var>new rule</var> is a syntax error, <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.
  <li>If <var>new rule</var> cannot be inserted into <var>list</var> at the zero-index position <var>index</var> due to constraints
  specified by CSS, then <a>throw</a>
  a {{HierarchyRequestError}} exception. [[!CSS21]]