[cssom] LinkStyle.sheet to return CSSStyleSheet instead of StyleSheet?

[saschanaz]

https://drafts.csswg.org/cssom/#the-linkstyle-interface

The associated CSS style sheet of a node is the CSS style sheet in the list of document or shadow root CSS style sheets of which the owner node implements the LinkStyle interface.

interface mixin LinkStyle {
 readonly attribute StyleSheet? sheet;
};

The sheet attribute must return the associated CSS style sheet for the node or null if there is no associated CSS style sheet.

The prose says sheet returns a CSS style sheet but the IDL says it returns a superclass instance (StyleSheet). Can we fix the IDL to return CSSStyleSheet?

[tabatkins]

While the IDL is technically correct, I see from your linked issue how it can cause some problems. I agree that we should be treating the StyleSheet/CSSStyleSheet distinction as a legacy mistake; the .type attribute is already fixed to always return "type/css", after all.

I'm fine with this change. Pinging @emilio.

[upsuper]

Maybe we should just merge the two types and make one an alias of the other?

[emilio]

Maybe we should just merge the two types and make one an alias of the other?

Well, that's observable I guess, since the prototypes would change, so that's a bit riskier... But yeah changing the IDL here should be harmless.

[tabatkins]

Yeah, merging has side effects. Probably fine, but there's little value in doing so, so eh.

[tabatkins]

(Feel free to merge #3313 yourself, @emilio, or Agenda+ it if you feel like you want review.)

[upsuper]

The main reason I brought merging is that, it's not clear what should be returned from StyleSheetList.item and StyleSheet.parentStyleSheet. If we admit that there's going to be only one type of StyleSheet, shouldn't they all return CSSStyleSheet?

[tabatkins]

Yes, presumably we should update all the IDL to return CSSStyleSheet instead.