An essay on W3C's design principles - Contents
Bert Bos's 2000 Treatise (published in 2003) is a must-read for anyone involved in developing any kind of format. "This essay tries to make explicit what the developers in the various W3C working groups mean when they invoke words like efficiency, maintainability, accessibility, extensibility, learnability, simplicity, longevity, and other long words ending in -y."