Campaign Monitor has completely updated it's guide to CSS support in email. Although there was a four-year gap between updates (and this thing has been around for 10 years!), it's continued to be something I reference often when designing and developing for email.
Calling this an update is underselling the work put into this. According to the post:
The previous guide included 111 different features, whereas the new guide covers a total of 278 features.
Adding reference and testing results for 167 new features is pretty amazing. Even recent features like CSS Grid are included — and, spoiler alert, there is a smidgeon of Grid support out in the wild.
This is an entire redesign of the guide and it's well worth the time to sift through it for anyone who does any amount of email design or development. Of course, testing tools are still super important to the over email workflow, but a guide like this helps for making good design and development decisions up front that should make testing more about... well, testing, rather than discovering what is possible.