Review consistency of the `<layer- checked`> (and `<extent checked>`) attribute with `<input checked>`

[prushforth]

In reviewing how our elements' design relates to the Web Platform Design Principles, we're told to ensure consistency of attribute use with existing attributes of the same name.

We should review checked to ensure some consistency with the API of the <input checked> attribute. That said, our use of checked is a bit different from that of <input>, but it does have similar UI implications.

The checked attribute seems to be one of the few counterpatterns that is listed that should not be kept in sync with the IDL attribute (property).

[yhy0217]

Resolved in #887

The checked attribute seems to be one of the few counterpatterns that is listed that should not be kept in sync with the IDL attribute (property).

The current implementation follows the pattern, the checked attribute is kept sync with the IDL attribute.