Not much has happened in the past 10 years .. still, there are a few useful ressources to the current state of HTML mail.
- Mailchimp: Email Client CSS Support - a comprehensive list of support in browser-based, desktop and mobile mail clients
- Email Standards Project - the once (more or less) standard reference; but seems to be somewhat outdated
- Campaign Monitor: CSS Support Guide - very detailed CSS support overview for a number of major mail clients
- - well-done, up-to-date (2017) and very complete guide, but with a rather bad index (ie. none)
- Litmus.com: A Bulletproof Guide to using HTML5 and CSS 3 in Email - Litmus.com being one of the major testing sites for HTML mails (similar to the mail ACID test)
- Litmus.com: Understanding GMail and CSS: Part 1