Web Standards
Web sites should be built to web standards.
In every industry, there are good and bad ways of doing things. It isn’t always obvious when standards slide. A house that isn’t built to building regulations may stand, for now. Similarly, a web site that isn’t up to code may look okay to you, but there may be underlying problems that wreck it for other users.
The World Wide Web Consortium (W3C) is responsible for the industry standards in web development. Every web site should use code that meets its standards. Minimally, that means that the code should be valid (if you have a site, but don’t know whether it’s valid, why not run it through the W3C validator to find out?).
Every site that I build meets web standards, with semantic mark-up and full separation of style and content.