[css-aliases] custom selector arguments

[bkardell]

When we originally discussed this, we didn't really have MutationObservers. It does seem that MutationObserverInit would achieve similar pieces to what we were looking for --- almost (I'll open another issue about the pice where it doesn't).

It seems like having the registration of a custom selector take a mutation observer init (maybe with sensible defaults?) would be a good idea.

[tabatkins]

Ooh, hooking mutation observers directly like that is probably a good idea.

I think the default of "any change to descendants" works fine.