Clarify guidance for onfoo attributes

[cbiesinger]

https://w3ctag.github.io/design-principles/#always-add-event-handlers says to add an onfoo attribute to the event handler of the element.

I am interpreting it such that if I have an HTMLFooElement that has an onfoo handler, I should add onfoo to interface HTMLFooElement.

However, WICG/fenced-frame#144 (comment) seems to say that such event handlers should instead be added to the global event handler list (interface mixin GlobalEventHandlers, at least in blink).

I am not sure if I am misreading the guidance and/or the discussion thread but it would be great to clarify what is the right thing to do here.

cc'ing some people from an internal discussion and from that thread:
@domfarolino @domenic @dbaron

[cbiesinger]

Hmm, looking at the document again, it says:

For consistency, if the event needs to be handled by HTML and SVG elements, add the event handler IDL attributes on the GlobalEventHandlers interface mixin, instead of directly on the relevant element interface(s).

Maybe "HTML and SVG" should be "HTML or SVG"?

[domfarolino]

Maybe "HTML and SVG" should be "HTML or SVG"?

I think you have the right interpretation. I'll submit a PR to disambiguate this in the design principles, and ping other editors to weigh in.