Thats a lot of effort compared to just plaintext that not only need none of this but also looks more professional, saves time and bandwidth.

The only people who care about HTML mails are scammer and marketing.

As a reader (and sometimes sender) of emails, I don't know why wanting my emails to be formatted when I'm reading them, so that some text is bigger than others makes me a scammer, but ok. Personally, I think it's quite nice when the 2fa email has the code in giant font so it's easier to pick out.