A generic 'alternative' mechanism for otherwise unknown tags seems
like a good idea if we're going to continue to invent new tags.
(If you don't understand <FROB>, then present this instead).
I can easily imagine browsers quickly updated to deal with <ALT> even
if they don't deal with any of the extensions.
HOWEVER, it makes a lot more sense to have the alternative tag come
before or around the thing it is an alternative to:
<ALT FEATURE=EMBED>This is what you present if you <EM>don't</EM>
understand <EMBED%gt. <EMBED>This stuff could be ignored
by any user agent that knows about ALT even though it doesn't know
about EMBED or NOEMBED or anything else</EMBED></ALT>
Well, so it's more intuitive for authors to put the alternative after
instead of before:
<ALT><EMBED>This is some stuff that is embedded.</EMBED>
And this is what you do if you don't understand the tag that comes
right after ALT.</ALT>