jsFiddle is the perfect solution; it provides a custom environment (based on user selections) to test (or fiddle with) your JavaScript, HTML, and CSS code right inside your browser. A quick tour ...

All you have to do is store your HTML, JavaScript, and CSS files, and then share a link ... adds zooming and region commenting to images ...

The new release brings improvements to JavaScript heap profiles, CSS code formatting ... the semantic highlighting of JavaScript source in HTML files with what is seen in normal .jscode files.