Two reasons: (1) the SGML standard, and (2) existing implementations.
SGML attributes just don't contain element markup, and web browsers
seem to agree. Most of them choke on > in side an attribute value!
> I can possibly see reasons relating to parsing why this might be,
>but it would be nice if one could provide something more than
>plain text as an alternate to an image - though I suppose the <FIG>
>element will take care of that.
Yes, it would be nice. We'll see whether folks like FIG enough to
support and use it, or if something else comes along. With Arena
hitting the streets, the whole HTML+ debate will soon surface again.
By the way... everybody on this list should drop by
comp.infosystems.www.providers and watch the folks out there debate
the issues. Some of the debate is just the eternal
semantic/presentational markup debate (about which the HTML 2.0 spec
says, subtly, "who cares?"). A lot of the discussion blames the HTML
designers for features that come right out of SGML.
But some of the debate is about folks honestly reporting
success/difficulty applying HTML to their information systems. We
should wade through the noise now and again just to stay in touch with
these folks.
Dan