Compress and gzip your CSS and JS.
CSS
Codepen+
A powerful online editor supporting HAML, SASS, Coffeescript, and more.
HTML5 Please+
Look up advanced HTML5 and CSS3 features and find out which browsers support them (hint: not IE).
CSS Selectors+
Brush up on your CSS selectors with live examples.
Ribbon Builder+
Build a fancy ribbon with nothing but CSS.
Layer Styles+
Create CSS3 styles using the Photoshop Layer Styles UI right in your browser.
Pea.rs+
Common patterns of markup and style for various elements such as tables, lists, etc.
