Outlook 2003 switched to the IE renderer, then for what I consider to be terrible reasons they switched back to MSO in 2007, and still use it in Outlook and even Windows Mail now. (The reasons I recall were bugs in the editor and security—.)

For people that aren’t in the know: the MSO engine is basically a very incomplete and quite buggy implementation of the HTML 3.2 specification. From where I stand, it looks like the entire thing is a binary blob that essentially hasn’t been touched at all for over twenty years. Literally.

Microsoft switching back to MSO breathed new life into an entire industry of people with esoteric, hard-won knowledge about the eldritch affairs of HTML for email. Without it, HTML for email would not be such a disaster. (Sure, there are other awful email clients, but Outlook is by far the most terrible that is used much these days. If it had tightened its game, other bad clients would have been much more like to fix theirs too.)

Isn't Office for Mac a completely diverged fork from an old Windows version of Office? I remember some blog posts from the understaffed Mac Office team stating that much several years ago. I don't know if they rebased it in the meantime. The posts read as if the big Office team was focused entirely on Windows and everything else was done by understaffed teams struggling to keep up.