Conditional application of style hints

There needs to be a way to tell the browser, "only apply attribute X if
you can apply attribute Y too."  Otherwise we'll run into situations where
documents will be unreadable because a particular browser (perhaps due to
platform limitations) can only present half the style hints, and the half
that *are* presented don't make sense without the others.

For example, I was playing around with style sheets in Arena 0.97 and
tried to put a background image in my document as described in the draft
spec.  "back.image" doesn't seem to be implemented yet.  Unfortunately,
font.color is, and my colors assumed that the background image would be

I'm not sure what the best syntax for this would be.  Maybe something like:

hasred := em.lightred:	font.color = #FF0000
hasred -> em.darkred:	font.color = #800000
hasred -> (a(em.lightred, (em.lightred(a: font.color = #00FF00

Where we allow an additional name to be assigned to a style element, and
allow other style elements to depend on the success of the first one.  Sort
of a dependency tree, if you will, which I think is an appropriate way to
think about this problem.  The above example would cause <em class=darkred>
to produce dark red text, and <em class=lightred><a> to produce green text,
only if the browser is able to render <em class=lightred> as requested.

A slight extension of this would also allow fallback behavior to be
specified.  For instance, if you want a black background on machines that
can't display your dark background image, you might do:

          didbg := *: back.image = "http://www.xyz.com/mygif.gif"
!didbg -> didbg := *: back.color = #000000
 didbg ->          *: font.color = #FFFF00

I'm sure someone else can come up with a better syntax!  Does anyone agree
that this is something that needs to be addressed?

An aside: when URLs are specified in a style sheet, what should they look
like?  Just the raw address?  The angle-bracketed URL: format?  Can they be
relative, and if so, are they relative to the URL of the original document
or the style sheet's URL?  That should all probably be addressed in the