Add HTML elements

The following elements are in the HTML Living Standard but are not represented here.

- [ ] [hgroup](https://html.spec.whatwg.org/multipage/sections.html#the-hgroup-element)
- [ ] [slot](https://html.spec.whatwg.org/multipage/sections.html#the-slot-element)
- [ ] [autonomous custom element](https://html.spec.whatwg.org/multipage/custom-elements.html#autonomous-custom-element)