See http://en.wikipedia.org/wiki/Html

Originally, before Microsoft stuck its d*ck beak in, as it has so many times before, stuffing up the normal processes as usually internationally agreed, it was JUST ONLY EVER ".html".

Of course, DOS, and the other Microsoft OSes using "8.3" formats was hopelessly stymied, so the ".htm" became a default (as far as I can tell, totally undocumented!) equivalent worldwide.

So then MickeyCrap 're-invented', as is its normal wont, '.html' as 'something Vista special'...!!!!

Of course, I may be just a little biased...