The way a mail reader chooses to display such an embedded message varies from one program to another. Some like to show them as “attachments” at the bottom of the screen while others “inline” them after the enclosing message, and yet some–less MIME-capable–just show them as uninterpreted text.
Using a Content-Disposition header, there is a way to request the receiving mail program to always inline them, but WebTV doesn't currently specify this, leaving it up to the receiving mail program to do what they think best.
None of this has anything directly to do with HTML. MIME is the Internet mail standard that governs how messages should be structured. HTML can be carried inside a MIME message, but that’s strictly optional. None of this would be a problem if all MIME-capable mail programs implemented the specification to the letter, but they don’t always. WebTV usually tries to work around incompatibility problems with other mail programs, even when they’re the ones that misbehave, but there's only so much that can be done, ultimately it's up to the receiving program to decide.
Most mail programs are already able to handle embedded MIME messages reasonably well, but not all do. Computer users using problematic mail programs should contact their e-mail program's manufacturer and tell them about this. After all, they’re the only ones who really can fix it.
Ultimately, there's little you as a WebTV users can do to make your mail more compatible with a computer user's system–other than using only plain text and no HTML, which should be compatible with any mail program in the world.
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