It’s actually a bummer: you can’t use a <style> tag because some email clients don’t like them. Instead, you have to inline your styles in every element. the lack of :hover is just a side effect of that I think (although it plays out nicely here).

(While on it, can we also ban loading images from third-party servers?)

I'm confused, we're talking about browsers, and comment I replied to suggested 'email + interactivity' as a standard for minimal browsers. I wasn't suggesting adding JS execution to emails. (I don't even allow remote images personally.)