In particular, the use of something like:
<a href="...." inline=true>Alternative HTML</a>
would provide a much more flexible model. Any type that
is defined could potentially be inlined, and a viewer that was
not capable of inlining the document would have the option of the
displaying the alternative and/or giving the user a chance to
display it externally. Alternatives could even potentially
recurse, although I'm not sure that's a _positive_ feature.
Further attributes could extend the model even further:
<a href="...foo.au" repeat=10>Alternative HTML</a>
could allow background music, for instance.