https://w3c.github.io/csswg-drafts/css-nesting/#cssom-style

I think this would be nicer as

partial interface CSSStyleRule {
  attribute ObservableArray<CSSRule> cssRules;
};

This:

• Ensures that cssRules has all the nice Array methods, e.g. map(), filter(), etc.
• Removes the confusing item() method.
• Allows manipulating the CSS rules using normal array methods, e.g. rule.cssRules.pop(), rule.cssRules.splice(4, 1), etc.
• Allows completing replacing the rules list, e.g. rule.cssRules = [newRule1, newRule2].

You could optionally keep insertRule() and deleteRule(), but they would probably be less necessary. (Although I guess there's no other way to go from string -> CSSRule?)

/cc @tabatkins as someone interested in array-like design on the web platform.