Galen Framework : Automated testing of look and feel for your responsive websites.
Layout testing seemed always a complex task. Galen Framework offers a simple solution...
Autoprefixer : PostCSS plugin to parse CSS and add vendor prefixes to CSS rules using values from Can I Use.
Skeleton : Skeleton: A Dead Simple, Responsive CSS Boilerplate for Mobile-Friendly Development
BackstopJS : Breaking CSS is easy. Checking every responsive page element is hard. That’s why there's BackstopJS.
UnCSS : UnCSS is a tool that removes unused CSS from your stylesheets. It works across multiple files and supports Javascript-injected CSS.