Complying with the standards (RFCs, IETF documents etc) instead of doing things your own way and causing pain to people.

Examples: in the WebBrowser world, InternetExplorer is NOT StandardsCompliant. Mozilla is. However, InternetExplorer is big (90% of all the web maybe?) so its intracies and syntax have become just as important to designers who want their pages to look good and read well.

As an old adage goes (roughly), the nice thing about standards is that everyone can pick one for themselves.

See RealWorldWebCompliance for some of our notes on how to make your web pages look good in everything.