I've found I really enjoy using webflow (a YC alumnus I believe) for responsive front end design. I literally design all my templates and components in their app now, export the raw HTML/CSS, and add any templating or client logic if needed. It's fun as hell. I also find looking at the generated CSS files helps me internalize CSS stuff I wouldn't otherwise have time to dig into. I know some folks look at me aghast for not hand coding my front end and I couldn't honestly care less.

I don't think it would work for everyone or every app, but for me it's been a dream come true.