Depends on what you mean by "backwards compatible." Older
browsers represent the "©" entity as "©", but that
hasn't stopped newer browsers from implementing it. Nor
should it, although it raises some very, very interesting
questions about whether © represents an official
copyright notice. Older browsers ignore tags such as <BODY
BACKGROUND>, which is to say, they represent them as a
uniform grey or beige background. In the grand scheme of
things, backwards compatibility is often something of a
give-and-take issue, and if style sheets can prove their
worth, I think that such a small backwards-incompatibility
is permissible.