Getting HTML emails to look good across all email clients can be tricky, but when the email has to be responsive, things can be downright brutal. Because some clients like Gmail and Yahoo strip out media queries or css in the head, it causes emails to not perform well across all platforms.
I use a mobile-first approach. This way when the css in the head is stripped out, it does not affect the layout of the email. My method has been tested with Litmus and looks great across all email clients.