[css3-values] Issue 18 proposal - defining an always-invalid URL

Issue 18 on CSS3 Values hasn't been resolved yet:
http://dev.w3.org/csswg/css3-values/issues-lc-2012.html#issue-18

To summarize, attr() needs to return a "default value" for each type,
for when the attribute can't be parsed as the type or when it's
missing.  For URLs, we'd like the default value to be always-invalid,
or never-resolvable, so that it fails no matter what context you're
using it in.  (It can't, for example, resolve to some UA-defined HTML
404 page, as that might be interpretable by some future CSS property
that, say, de-magic-ifies iframes.)  However, there is no such URL in
existence so far.

We propose just defining it to be the url "about:invalid", and then
subsequently getting that URL added to the about: registry.

Any objections?

~TJ