Questions such as these are often asked from time to time.
Please see this
other thread as an example.
It's obviously very difficult to pigeon-hole groups and artists into specific genres, of which
Progressive Rock is one. As for the specifics mentioned here, the
'Dead are very much a psychedelic group above anything else. As such, whilst they aren't considered 'strictly' Prog, they most certainly do orbit (albeit at quite some distance ...) around the 'Sun of Prog'. And it's because of this that they are occasionally featured here on S.A. And I do stress occasionally. Prog Trivia - they were mooted to tour with
Yes a few years ago.
Likewise as far as the wonderful
Led Zeppelin are concerned - there is most definitely some overlap in their music, which consists of more than their use of the Mellotron.

Lastly, as for
Elton John, he was seriously considered as the front man for
King Crimson during the recording of their 1970 '
The Wake Of Poseidon' album. Seriously.
This interesting topic will no doubt continue to re-surface again.
Darran