[hollerith]

It's good to be vigilant against hypocrisy, but there is such a thing as being too focused on possible hypocrisy.

What HN does, i.e., allow comments to contain a couple of carefully-chosen formatting options, is not a realistic option in email whereas because of a history of decades in which email clients could render only plain text, asking email senders to send plain text is a realistic option at least in some situations.

In other words, HN's designers were not restricted to a choice between plain text and full HTML, but in email we basically are (because there is no central authority in charge of email).