This function was implemented in a few classes but is a common element
in all form associated elements and the functionality should be there.
With these minimal changes we get to implement 4 idl functions for free.
This porting effort makes it pretty clear we will want a UTF-16-aware
GenericLexer. But for now, we can actually make ASCII assumptions about
what we are parsing, and act accordingly.
This change — part of the HTML constraint-validation API (aka
“client-side form validation”) — implements the willValidate IDL/DOM
attribute/property for all form controls that support it.
Corresponds to part of https://github.com/whatwg/html/pull/9841 and then
https://github.com/whatwg/html/pull/11047
Adding `Auto` as a type state feels a little odd, as it's not an actual
type allowed in HTML. However, it's the default state when the value is
missing or invalid, which works out the same, as long as we never
serialize "auto", which we don't.
The popoverTargetElement seems to be one of the only cases of a
reflected Element? attribute in the HTML spec, the behaviour of which
is specified in section 2.6.1.
Buttons can't actually toggle popovers yet because showing/hiding
popovers is not implemented yet.
