jsFiddle lets developers play with the three core elements of Web development. Check out this powerful application for rapid prototyping and testing.

Built on top of GitHub’s Electron framework for cross-platform desktop apps, Quark is a general purpose tool that leverages JavaScript, CSS, and HTML.

"The Web is built on HTML, CSS, and JavaScript, so SproutCore is in JavaScript and it leverages the built-in classes of JavaScript.

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What is CSS in JavaScript and why is it suddenly popular? Learn how it works and get introduced to some of the leading CSS-in-JS frameworks.

jQuery UI is another option for building HTML and JavaScript-based application interfaces. It's completely open source and has the advantage of being directly compatible with jQuery, jQuery Mobile, ...