User interaction and details element

There's something I don't understand about the <details> element, I hope
this is the right place to ask because it could also be fixed in the future.

The spec says, and it's a behaviour commonly honored by UAs supporting
the detailselement, that the "open" boolean attribute is to be set or
removed when users click on the summary element.

As far as I know there aren't many situations where user interaction
actually changes the DOM. Namely, when users click on checkboxes, the
control state changes but no "checked" attribute is set on the element
itself, though the checked IDL property changes (of course this is valid
for all of the form controls, but checkboxes and radio buttons are closer
to this case as checked is boolean).

This is valid for newer tags, too. Visual UAs are expected to show a
control bar for <video> elements when the controls attribute is present.
This bar can also contain a volume control with the option to mute the
video. But the spec does not say that UAs have to change the "muted"
attribute, also because its presence defines the state of
the defaultMuted IDL property.

Of course nothing bad can derive from this, but it's a sort of
inconsistency. If it's just a matter of CSS, it would be more consistent to
have an ":open" pseudo-class for it, corresponding to the open state. And I
cannot believe it hasn't been done for legacy issues, because nobody stood
against new pseudo-classes like ":required", ":valid" and so on.