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 ...

Web-based technologies including HTML and JavaScript are less rigid with regard to syntax than languages such as Java or Python, which makes them easier for new developers to learn. HTML, CSS and ...

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.