Event handler attributes language is not very clear

[bzbarsky]

https://svgwg.org/svg2-draft/interact.html#EventAttributes says:

For every event type that the user agent supports, SVG supports that as an event attribute

But "event type that the user agent supports" is not defined. Events can be created and fired with arbitrary types, and UAs support that.

This should probably actually define the set of attributes that should be treated as event handlers, possibly by reference to other specifications (e.g. HTML).

[AmeliaBR]

Looks like this is a duplicate of #467. Closing this and copying over the comment & Agenda+ label so we can keep all the discussion in one place.