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
