It really depends on the DOCTYPE string that is set in your HTML pages. If your DOCTYPE is set to a version of HTML before 4 then you are ok. Versions of HTML after 3.2 deprecated the font tag (and a few other tags). And if you are using XHTML forget it. Any font tag is invalid in XHTML. In XHTML CSS is the only way to go.

Here are lists of elements deprecated in html 4 and in xhtml.

If you are not using a DOCTYPE then all bets are off. Its a crapshoot on how browsers will render your page.