Re: iframe@srcdoc

I thought that someone would have answered like that, but currently there
are no user agents which fail to support user agents and those which do,
lack the support willingly (such as some search engines). The evidence is
that in XHTML documents the iframe element has "content model: empty".
Modern browsers, i.e. those which support XHTML documents (but even much
older ones, see IE versions < 9) don't need fallback. In addition to this:
 - a proper fallback could actually work as initial content, given the fact
that fallback sentences such as "your browser doesn't support iframes" or
so on are useless to the common user and several guides discourage it in
favour of a link to the embedded resource.
 - the question could be resolved changing the absurd rule that initial
content takes precedence over embedded content: an iframe having both
content and a src attribute could display the embedded resource, so that
the initial content is shown only when a src attribute is not defined or
when it maps to a specific value (e.g. a document fragment matching the ID
of the current iframe). So the content would provide a fallback unless
explicitly exploited (the 2 functions overlap fairly well IMO).