It does handle HTML, but it'll look different from what people reading email in Firefox etc. see. So just like with eww.
(Also, Outlook has MS-specific niceties like replying directly to Teams messages from within an email without having to start up Teams, which I assume they made as a workaround for how long it takes to start up Teams and pick the right organisation and wait for it to tell you to log out (since your account is still connected to that one organisation that long ago invited you for a single meeting and only org admins can remove users) and then hit reload since that sometimes works and then pick the right organisation again and then reply "OK" and then quickly exit Teams before your computer runs out of memory and grinds to a halt.)