Well, most people would direct you to media queries. I highly recommend you take a browser, build for the biggest screen and then start decreasing the viewport size to a point where you think your app starts looking ugly. That's your first breakpoint. Then you write a media query starting from that breakpoint so your site looks good again. Then you shrink the viewport again and repeat the previous steps. Do that until you reach the smallest size you want to support (for example medium sized mobile phone; or maybe small laptop screens as you claim to have covered mobile and tablets already). You can read about that technique in detail on smashing mag's blog: smashingmagazine.com/2012/03/device-agnostic-appr…