Sorry, no idea on that one. WebTV might've been doing something strange, like putting in character codes or non-breaking spaces instead of normal spaces, or for some reason trying to parse the HTML itself before posting.

Currently, you can't readily enter raw HTML in the "advanced" editor (it doesn't work well for that unless you always remember to use the SOURCE button), but with the Normal editor, quite literally whatever you enter should be directly parsed by your browser later.