Codepen

CodePen is a playground for the front end side of the web. CodePen is an HTML, CSS, and JavaScript code editor in your browser with instant previews of the code you see and write. It allows easily to explore web technologies with just your browser (no other development environment needed). CodePen can be found at http://codepen.io/. The service has limited free plan and commercial plans. It is also possible to test it without registering.

Here are some examples to experiment with: Animated Clock and Shutter Test (links to more on home page).