Wait, wait, wait...
I think the *HTML* standard can have in it what it's willing to
include in a HREF or IMG SRC=; e.g.,
in that standard, you can say that you allow
<A HREF="reference" ...>
where a "reference" is either a:
URL
local file designator
exec link
relative URL-like-thing
etc.
Not all things that might appear in a *HTML* standard as 'allowable in
a link' should appear in the *URL* standard.
In particular, applications that require *URL*s but can't deal with
'exec links' and other things might be archie-like-things, resource
discovery programs, internet directory services, etc.
Just because it isn't in the URL standard and looks like a URL doesn't
mean you can't standardize it.