From the W3C MarkUp Validation Service Help section:
Why should I validate my HTML pages?
One of the important maxims of computer programming is: Be conservative in what you produce; be liberal in what you accept.
Browsers follow the second half of this maxim by accepting Web pages and trying to display them even if they're not legal HTML. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal construct; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a mangled mess, or even crash.
That's why you want to follow the first half of the maxim by making sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validators.
As of 2002-12-12, I've been working to move my main pages to XHTML 1.0 validity and also planning a site-wide upgrade to use stylesheets for appearance control. Although I currently consider it to be at the top of the priority list to be compliant with standards, I don't actually want to put this:
on every page. So I'll leave it here under the caveat that it certainly applies to this page and the page that got you here, but not necessarily everything else you can get to. You can yell at me if I have an invalid page somewhere near the surface; otherwise, await the unveiling of the final design.
If you actually care that much about the validity of my pages to test them and subsequently yell at me, I use Jim Ley's "Is This Page Valid?" favelet to check pages. Right-click and bookmark the previous link, stow it in your Personal Bookmarks, and enjoy. Be warned that the favelet won't work for pages that require you have the right cookies set. I'm now running a local copy of the validator to (eventually) work around the problem.