[css-typed-om] TypedOM uses WebIDL setters with non-undefined return values

[emilio]

CSSTransformValue has:

setter CSSTransformComponent (unsigned long index, CSSTransformComponent val);

CSSUnparsedValue has similar code. That interestingly caused issues (warnings) on the Gecko codegen (bug 1981764).

The return value gets ignored in other implementations as well, and I think it doesn't make sense, since the behavior of the assignment operator is to return the right-hand-side, so if you were to return something else from webidl it'd be... odd?

I don't know how/if you can get the actual native function for the setter to call it directly and check its return value, I don't think you can... So is the return value of a setter observable in any way?

I think this is most likely an oversight, WebIDL setters should probably all return undefined, and TypedOM should use that... @tabatkins @andruud, am I missing something?

cc @saschanaz @janvarga

[tabatkins]

Oversight, it's kinda weird that the setter needs to return undefined but actually does return something.