Including, apparently, the people who created the web site, and who use bold text and different font sizes to format the headlines, colors to indicate features which are supported and not supported, and an image of a "plaintext certified" stamp.
> "But if plaintext is so good, why is this page written in HTML?"
> This is a reference document, not an email, you twit.
They could have made this site plain text and followed all of their own guidelines, but they didn't. Instead, they wanted to make the site look prettier to get their message across with additional formatting plain text can't handle. They're reducing accessibility and increasing security issues just for their own superfluous desires.