Unless the user specifies otherwise, our default is to send both plain text and html (if not pure plain text is sent). Mutt can be sett to either use the plain text part, OR (apparently), to convert the html part and show that. https://wiki.archlinux.org/title/Mutt#Viewing_HTML
Bug 1727493 Comment 7 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Unless the user specifies otherwise, our default is to send both plain text and html (if not pure plain text is sent). Mutt can be set to either use the plain text part, OR (apparently), to convert the html part and show that. https://wiki.archlinux.org/title/Mutt#Viewing_HTML