Standard Standards.
There is no excuse for not observing the rules of the road. On the Internet, the rules are standards, and on the Web the standards come from W3C. It's thanks to the W3C I have free tools to verify both my xhtml, and my CSS.
Thanks partly to my laissez faire approach getting everything to comply with the standards was mildy icky. There are still a few little insignificant things, but I've done it, I'm finally compliant.
Gives me a warm fuzzy feeling. And a feeling of moral superiority, which I'm sure will give rise to riteous indignation whenever a site fails to work with konqueror, or some other !IE browser.