I'm surprised to find no mention of MJML on this thread - it's more of a developer tool than a user facing one. You get to design your emails using a simplified markup language that resembles HTML, and it converts it into minified HTML that works pretty well on most email clients. Kudos to the guys behind it!
I think that https://www.caniemail.com in many cases would be a timewaster. Because you spend your time exploring each individual CSS/HTML feature availability and later on realizing something can not be done in fully crossbrowser/cross-email-client way.
Instead you could just accept the limitation of MJML and start coding the final version right away.
