Re: [dom] Remove feature detection from add() and add supports() (#123)

So another thing I think we want to change is that "supported tokens" is an aspect of the `DOMTokenList` and not related to the attribute's local name. That requires changes to how "supported tokens" is defined, to be more in line with other associated slots. And it requires how that is referenced elsewhere.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/pull/123#issuecomment-163354104