You can only create 4095 style rules with this method. After that, it throws an "Invalid argument" exception.
The same limit applies even if you append style rules using the cssText property.
BTW, the following code is much faster than repeatedly invoking addRule for each new rule.
var ss = document.createElement('STYLE');
document.documentElement.firstChild.appendChild(ss);
ss.cssText = '(a very long list of style rules)';
http://msdn2.microsoft.com/en-us/library/ms533698.aspx