After much googling around (I was fascinated by this question) and much reading of various W3C documents here and there, I can say with about 97.3% certainty that the W3C has never drafted a recommendation that standardized file extensions. Most of their recommendations include URI examples that use the .html extension and the site itself appears to use .html extensions:

The real story of why .htm and three letter extensions were ever used is told in a round about way here: .

Given the history of filename extensions, I can see how someone might think that a three-letter extension is required (and maybe--for some strange reason--the server's settings do require html to be served as .htm and .html files are served differently). Whatever the case, the W3C doesn't recommend .htm as a standard.

Jody Tate
Web Developer - UW Network Systems

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