Add HTML elements

[scottaohara]

The following elements are in the HTML Living Standard but are not represented here.

• hgroup
• slot
• autonomous custom element

[scottaohara]

This issue will be picked up once #226 is completed.

[scottaohara]

copied from ARIA in HTML issue regarding custom elements:
Per: HTML issue 4658 and ARIA issue 984, it should be specified that a custom element may have no corresponding role (map to generic) or may have a role set by ElementInternals.

If the role is set in that way, and the custom element doesn't merely act as a container for child elements (light or shadow dom) with their own roles, then the listing for the custom element would need to indicate that it either maps to generic, OR the role set by ElementInternals, and to reference THOSE roles for appropriate mappings.

[carmacleod]

Can we create a separate issue for hgroup so that it is not held up waiting for autonomous custom elements? Is hgroup "Not mapped" everywhere? Does it need to wait for the "If mapped" PR #226?

(For context, I'm hoping to point to hgroup (and slot) from the html spec, but at present, they're missing from html-aam). :)

[scottaohara]

I do not think #226 should be a blocker here anymore, per my comment today in that issue's WIP PR.

hgroup is not mapped anywhere and may well continue to be unmapped (though impact the heading levels of contained elements) depending on how that gets reworked in the HTML spec.

slot is not mapped, nor should it be.

autonomous custom elements are either role=generic or mapped to the role declared via its ElementInternals.

form associated custom elements should map to the form-related role declared via its ElementInternals.