And HTML validator is only good for pages that are public and "semi-static" (ie, don't rely on sessions to display ALL of the contents).
So when I have a choice of firing up IE to check things out or logging in (or creating a session) with Firefox, saving the HTML to a file on my disk and then uploading it to the validator - I'll pick the former, and spend the rest of the saved time doing more productive things.
Actually that's not true ... it's all about what tools you're using to validate. If you install the Firefox extension "Web Developer Toolbar " one of the options right on the toolbar is validation and in addition to standard HTML and CSS validation, "validate local HTML" and "Validate Local CSS" are options. All you have to do is look at the page and hit a button - it will upload the LOCAL HTML output exactly as you see it in your browser and validate it. Simple. You don't have to worry about sessions or anything, just go to the page and hit a button.