schemes and relative URLs

According to section 3 of RFC 2396,
an absolute URI must begin with "<scheme>:"
and a relative URI must not.

It also says that the authority part of a URI must be preceded by "//".

So that would mean that something like:

    href="http:foo.html"

is not legal, though I believe some browsers treat "foo.html"
as a relative reference (as the author likely intended)
and some as a site name.

Assuming my interpretation above is correct,
the question is, can your "http://validator.w3.org/check?uri="
service be taught to notice such invalid URIs and report them?