RE: any difference between *.htm and *.html extens
Not at all. The three character implementation is more prevalent on Win-based servers such as IIS, but in reality either is fine. One case where I've personally used different extensions is when I'm indexing web pages for searching and only want certain pages indexed ... I will make some pages HTML and other HTM. That way I can make the search work on only the pages I have made with an HTM extension or vice/versa.