ProtoFluid - HTML5 and CSS3 Web App Prototyping
Test your @media queries and web app on various screen sizes
CSS3 Please! The Cross-Browser CSS3 Rule Generator
You can edit the underlined values in this css file but don't worry about making sure the corresponding values match, that's all done automagically for you.
Web Font Specimen
See how type looks on the web
Proudly Announcing: Modernizr | FarukAt.eş
Modernizr is a small JavaScript toolkit that uses feature detection techniques to determine whether or not the current browser is capable of any of the following features from CSS3 and HTML5.