I stumbled upon Validator S.A.C. through a link off Shaun Inman's site. It seems worth the download, if you're concerned with quickly validating your HTML/XHTML and find yourself at the W3C validator often. It's fairly versatile, for a simple app:
Validator S.A.C. can validate local files, as well as pages on local web sites. Validate using the stand-alone Favelet, drag and drop files and URLs onto the application icon, or simply use File > Open. Validator S.A.C. can also be setup as a Web Service, allowing users to validate over the local network.
After some searching, I found that you could install the W3C HTML Validator directly in OS X. However, Validator S.A.C. seems much more convenient.
Now, let's see some Markup/CSS combo action and then we're talking.