Micoo : Micoo is a pixel based screenshots comparison solution for visual regression test
BackstopJS : Breaking CSS is easy. Checking every responsive page element is hard. That’s why there's BackstopJS.
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...
* { CSS:resetr } : To help you find the perfect CSS-reset styles, CSSresetr lets you try all the best CSS Resets on a variety of web pages – even your own.
Diagnostic.css : Diagnostic.css is a stylesheet which allows the user to test for common errors in a page's markup.
Also : https://github.com/karlgroves/diagnostic.css
Instant visual diffing with CSS blend modes!
Simply enter the sites you want to compare. Locally hosted addresses and files also work (so you can see how your local changes affect current pages or previous versions).