You send me anything other than plain text in an email, and I don't already have a source filter for your address, it gets sent to a folder that I might look at if I have the time some day.
99% of important transactional email I get is HTML. Kind of nice to have your boarding passes and two-factor authentication emails easily visible.
Click this link to reset your password or some such.
Luckily, HTML only (or its bastard cousin: including a text/plain part that is horribly broken or just says "please enable HTML viewing") isn't seen that often around here.