Remove the "template" use case for hidden=""

[zcorpan]

Having thought a bit about #4904 I wonder if the "template" use case for hidden should be removed:

When specified on an element, it indicates that the element is not yet, or is no longer, directly relevant to the page's current state, or that it is being used to declare content to be reused by other parts of the page as opposed to being directly accessed by the user

https://html.spec.whatwg.org/multipage/interaction.html#the-hidden-attribute

This predates the template element. The template element is better suited to address this use case, for several reasons: hidden stuff is not inert, can fetch stuff, can submit forms, can execute scripts, can be rendered to the user if display is changed, document conformance requirements apply normally but in templates you usually want to have variable parts which quickly ends up violating document conformance requirements.