HelloSign: Everything IT requires and Developers love
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.