OK, now I found the document it is referring to. Nice domain name!
https://useplaintext.email/
I laughed at this one:
「"But if plaintext is so good, why is this page written in HTML?"
This is a reference document, not an email, you twit.」
I don't agree with the suggestion that *strong* and /emphasis/ is just as good as using a different font weight and slant, unless the email client does that for you. It sounds a bit like cheating? Or like a very reduced HTML subset?
Well, that would work for me…