Screenfly : Test Your Website at Different Screen Resolutions
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
Pa11y : Monitor the accessibility of your websites with pa11y-dashboard, and protect against accessibility errors creeping into your codebase.
tota11y : an accessibility visualization toolkit that aims to reduce the friction of a11y testing
Lighthouse : Lighthouse is an open-source, automated tool for improving the quality of web pages. You can run it against any web page, public or requiring authenti...
The mobiReady testing tool evaluates mobile-readiness using industry best practices & standards.