These apply to any elements that have some kind of open/closed state.
The spec suggests `<details>`, `<dialog>`, and `<select>`, so that's
what I've supported here. Only `<details>` is fleshed out right now,
but once the others are, these pseudo-classes should work
automatically. :^)
This matches if the element has a placeholder, and that placeholder is
currently visible. This applies to `<input>` and `<textarea>` elements,
but our `<textarea>` is very limited so does not support placeholders.
As noted in a9620d8784 we don't currently
set the target element so this does not function, so no tests. But it
should work once we have a fleshed out Navigables implementation. :^)
