We don’t generally think of CSS animations or transitions inside of email, or really any movement at all outside of an awkward occasional GIF. But there is really no reason you can’t use them inside HTML emails, particularly if you do it in a progressive enhancement-friendly way. Like, you could style a link with a hover state and a shaking animation, but if the animation (or even the hover) doesn’t work, it’s still a functional link. Heck, you can use CSS grid in email, believe it or not.
Jason Rodriguez just wrote Understanding CSS Animations in Email: Transitions and Keyframe Animations that covers some of the possibilities. On the supported list of email clients that support CSS transitions and keyframe animations is Apple Mail, Outlook, and AOL mail, among others.
Other things to look at:
- Email on Acid: The Ultimate Email CSS Animation Guide
- FreshInbox: Kinetic Email CSS Support
- Campaign Monitor: The Ultimate Guide to CSS (animation)
- The incredible trickery of camiah