Allow toggles to work on elements with existing behavior

[mirisuzanne]

The current toggle-trigger production says:

If the element already has existing activation behavior from the host language, this value does nothing.

I'm not sure how clearly defined the term 'activation behavior' is, but it seems to imply that toggles can't be applied to button elements.

I believe that's a mistake, and should be clarified. But I'm also curious how strict we need to be here. For example, the proposed popup spec allows popup behavior on any element, including those with existing behavior - but the existing actions (e.g. form submission) take priority. It seems like the toggle spec could take a similar approach?

/cc @dbaron who initially pointed this out.