Earlier quoted context omitted.
I use mutt and unfortunately in the corporate world you can't just ignore HTML or even HTML only mails. I pipe them to elinks, works 99.9% of the time easily.
That may be the case, but that still doesn't make HTML a part of email itself. It's using email to send/receive HTML.
https://datatracker.ietf.org/doc/html/rfc1341
HTML email is not merely an HTML file sent by email – it is the email.