Should built-in states be reflected in the API

[plinss]

Given the change in syntax, and that all custom states now have to be prefixed with '--', it raises the question of whether built-in states should also be reflected in the API.

e.g. if an element is being hovered over, should el.states.has('hover') be true?

This fits with Houdini's mission to explain existing CSS behavior.

Of course, the user should no be able to add or clear built-in state.

[tkent-google]

Yeah, we can extend the feature for existing built-in pseudo classes.

WICG/webcomponents#813