Using OmniWeb 4.5, you can set up a shortcut in the Preferences to easily validate the HTML of web pages.
Go to the Preferences and select Shortcuts. Add a new shortcut (click the "+" sign) and then double-click the "somewhere" name and rename it to validate@. For the "Destination URL," enter http://validator.w3.org/check?uri=%@.
To use the shortcut, just type validate www.somesite.com, hit return, and watch it use the w3c validator to validate the page.
[robg adds: A cool trick, but please, don't flood my inbox telling me how badly the macosxhints' page fails the validation check ... I know, and there's not a lot I can do about most of it myself! For fun, check microsoft.com!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030807172012941