I am a very big proponent of technology compliance standards and strive to meet such criteria in all of my work. That includes my site, which is 100% XHTML 1.0 compliant as determined by the World Wide Web Consortium.
If you make your website compliant, you can add the compliance link to every page of your site! The code snippet below allows for users to click the link to really know you are compliant.
The best compliance to go for is XHTML. This is because XHTML is a stricter and cleaner version of HTML. XHTML not only makes it easier for people to follow your HTML code, but makes it more likely to be displayed in multiple browsers. With many surfers using browsers such as Apple Safari, Google Chrome, Mozilla FireFox, or the multiple version of Microsoft Internet Explorer, writing clean strict HTML code is the best thing you can do for usability of your site.
You may be tempted to say that a majority of "normal" web users use IE anyways, and any one technical enough to use an alternate browser will be able to figure the site out anyways. But think of all the Mac users who use Safari or Opera browsers! Do you really want to keep the technically inclined and Mac users away from your site?
I know as a Experts Exchange Expert I would love if every piece of HTML code I looked at was formatted XHTML compliant. This would make it much easier to troubleshoot problems when users post their code.