Pretty sure somebody has already posted this here, but it's worth re-posting over and over again!
https://useplaintext.email/
Thanks to #SourceHut for putting this together!
@rysiek I used to be a hard-core believer in plain text but it's been a while. Nowadays assertions like "HTML emails are mainly used for marketing" and "the tradeoff isn't worth it" are to me statements about one's bubble rather than shared reality.
In a lot of contexts the difference between plain and rich text is much more important than a lot of techies realise.
@rysiek And the "you twit" at the end is just off-putting: admitting that some communication requires more than plain text with a strong assertion that it's a kind of communication that does not belong in an email, punctuated with an insult instead of attempt to understand at why people might in fact need that kind of communication in email.
@marcink the "you twit" thing was very clearly in the context of "this uses HTML because it is not an e-mail, but a reference document" bit, which I imagine is there because some Very Smart People were "arguing" that "but why are you using HTML here, then, if it is so bad? 
".
And in such a specific context it feels justified to me. But of course personal opinions are personal.
@rysiek I get that, but when this document reaches a new person this is likely to also be their question. Putting aside whether "if you really need rich text you're doing something outside of the allowed uses of email" is the right answer or not: try to imagine what kind of side message they're getting from that part of the doc.
This kind of thing is why the techie community is still largely viewed as abrasive.
