Re: Yet ANOTHER reason!
" I'm pretty sure the OP meant one that doesn't make any kind of outbound HTTP call when viewing the message."
that's one, but there are many things that style sheets can do that pose a potential problem. there's also HTML5 content (yes I really wanted to see that streaming video when I opened an e-mail) and things like that. But style sheets can have script-like behavior, too. They can get really large, and really complicated. And, of course, loading the style sheet across 'teh intarwebs' identifies YOU as the mail recipient, even if all it does is check to see that you have the latest version with a 'HEAD' request.
a style sheet can, for example, passively determine what your screen resolution is. Content that uses a particular style can then (theoretically) use this information to "phone home" that info on you. I forget the exact details on how it works, it has something to do with being able to manage auto-sizing column widths as one possible usage. I've actually worked on customer web pages that do this. Don't ask me HOW it works, it was confusing enough fixing the existing page so it would look right on a phone in portrait mode, or on a desktop or a 'slab' in landscape mode, with their varying aspect ratios and screen sizes [yes it works perfectly now!]. And I didn't have to change the style sheet - I embedded 'style' info into the HTML.
So using this information, indirectly determined from the style sheet setup, EVEN WITH SCRIPT TURNED OFF, it should be possible to 'nuke out' what some of the hardware is that you have on your computer. That doesn't even include font embedding or other potential danger items. There have been vulnerabilities with web fonts in the past, after all.
it's like a potential side-channel attack. You know, like Meltdown and Spectre.
seriously isn't the USER-AGENT bad enough in external HTML requests? Only now, it's e-mail spam doing this (in particular, spammed malware). And THOSE are the people who will leverage it.
icon, because, paranoia (again)