>I have used a couple web/page different authoring tools now. Some assign a default .HTM, and some assign a default .HTML to HTML docs. Then I start noticing on the web what's out there, and there's a mixture. Seems like there ought to be a standard for this?
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>I can see a point in using .HTM, it fits a 3-char extension suffix which might be a good idea for interaction with various software. Any comments as to which is preferred, or is there some subtle difference I'm not aware of?

There's no difference, except for the point that you mentioned- possible compatibility problems with software that expects a 3 character extension. I name my files with 3 chars for this reason, but beyond that, I wouldn't worry about it a bit.