It's not super clear what toggle() and attr() should do in sizes="". We should just ban anything but calc().
toggle() technically works, but since there's no inherited value, it'll just always select its first value. Fine to ban it.
attr() is technically only defined within the context of a property, which sizes isn't, so it doesn't actually work. Fine to ban it too.
I agree that it should be fine to just whitelist calc(), and we can expand the list in the future if needed.
Clarify that toggle() and attr() are invalid in sizes. Fixes #241