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.
