Since all URLs that are in HREF, SRC or ACTION must be escaped, and spaces are
among the characters escaped, it would seem syntactically possible to permit
multiple space-separated URL's as values to these attributes. Semantically, a
user agent would then have one or more alternative pointers to a resource in
the case where the first one(s) were unavailable.
The <A HREF="http://www.w3.org/pub/WWW/MarkUp/html-spec/html.dtd
ftp://ftp.ckm.ucsf.edu/pub/html/html.dtd"> HTML DTD </A> is widely available.
or
<FORM ACTION="http://my.session.server/transaction?do+this+for+me
http://generic.sesion.server/back_from_the_dead">
This currently doesn't blow up on both netscape 1.x and xmosaic 2.7.latest,
although the BASE must be set in the referenced document so that inlines' URL's
are not prefixed with the full contents of the HREF and correctly resolved.
-marc
-- Marc Salomon