Is there even a standard on the subset of HTML/CSS/… that is used in emails and their actual usage?

Because I seriously remember having frontend devs (yes) writing huges articles on how to design theses and it's quite close to reverse-engineering.
And I'm pretty sure that no client does quotes the same way.

And no this is not to yell in my mentions about the plain text vs. HTML emails shit, I have my opinions on theses and while not much extreme they are very settled.