[whatwg] hit regions: limited set of elements for fallback content

[Rik Cabanier]

The current spec for hit regions restricts what elements can be used as
fallback content [1]:

If any of the following conditions are met, throw a
NotSupportedError<http://www.whatwg.org/specs/web-apps/current-work/multipage/infrastructure.html#notsupportederror>
exception
and abort these steps.


   - ...
      - The arguments object's control member is not null but is neither an
      a<http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#the-a-element>
element
      that represents<http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#represents>
       a hyperlink<http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#hyperlink>,
      a button<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#the-button-element>
element,
      an input<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html#the-input-element>element
      whose type<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html#attr-input-type>
attribute
      is in one of the
Checkbox<http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#checkbox-state-(type=checkbox)>
       or Radio
Button<http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#radio-button-state-(type=radio)>
states,
      nor an input<http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html#the-input-element>
element
      that is a
button<http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#concept-button>
      .

Why is there this limitation?


1:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#hit-regions