Dear Tina:
I've asked this question myself a bunch of times and I get a different answer each time. The problem lies in the fact that different platforms use different file naming schemes. Old DOS systems, MACs, Unix & Windows are all different. Even the HTML editors like FrontPage and Dreamweaver use different defaults. I've been using both .htm and .html succesfully for over two years and I have never had a problem. A bigger problem is using captial letters in your file names, but that's a different story.