The details element

To the attention of whoever interested.
Does anybody know when the <details> element, along with its controller
<summary> and all references to them, were removed from the official spec?
And more important, why was it removed? Is it due to lacking support?
Because, IMHO, removing an element from the list of allowed elements is not
the best way to ensure that authors make use of it, thus driving vendors to
upgrade browsers and offer support. I mean, authors worrying about validity
issues will not use it at all. Like me, as I'm feeling the urge of removing
it instead of ensuring its functionality via a polyfill.
I hope the removal was a mistake. Evidence would be the fact that it is not
listed among the obsolete elements. Despite the section name, the list also
contains the ominous <hgroup> element, which in an early stage was meant to
group headings and subheadings in the form of <h#> elements poorly used.
Otherwise it could be due to the fact that it is basically a presentational
element, whose purpose is NOT to highlight a real "details" section, but to
have a widget with an open/close toggle behavior. But, were that the case,
why is the element still present in the nightly?
As a side note, for validator crew (and unfortunately this means only Mr
Michael Smith): the validator says that the element "is not supported
properly by browsers yet", but as a matter of fact it is a non-conforming
element now. Please update the error message properly when you have time.
Sorry for verboseness.
Andrea R.