CodePen is a web-based HTML, CSS, and JavaScript code editor that enables its users to experiment with code right in the browser. It is an open-source learning environment, displaying a live preview of the code changes. It can be used for testing out bugs, collaborating, and finding new inspiration. It works by allowing users to create pens, whichare sets of HTML, CSS, and JavaScript. Users can then display those pens on their profile, take feedback, and continue to edit those pens at any time.