If you have not installed the Web Developer extension for Firefox, DO SO IMMEDIATELY. It saves you hours of time on almost any web project. I use Opera as my primary browser, but I still go back to Firefox when I'm checking websites because Web Developer is so versatile. In its Tools menu you can choose to validate your page's XHTML and CSS (either online or locally) and you can check speed reports and accessibility standards if your page is online. It's much, much quicker.
You can add your own validators, too. I added one that I called Validate Site HTML, which validates your page and all the pages on your site that that page links to. It wasn't difficult to set up because the WDG already created such a validator; it was merely a process of connecting the dots.
By the way, I wouldn't trust too many markup validators, because it's hard to know whether or not they update their tools regularly. I know, for example, that Dreamweaver 8's internal validator says that in an XHTML 1.0 Transitional document,
<html lang="en-ca"> is invalid because it needs
xml:lang="en-ca" as well; the W3C and WDG validators don't care. (I don't know how good or bad Expression Web's validation capabilities are ... I usually code too valid to notice. ;)
Jonathon VS
Freelance Web Artist
www.jonathonvs.com