CodePen

CodePen is an online code editor built for front-end web development, letting users write and preview HTML, CSS and JavaScript directly in the browser without setting up a local project. Projects, called pens, can be saved, shared and embedded elsewhere, making the site a common place to demonstrate small web experiments and techniques to other developers.

It also functions as a social platform for developers, who follow each other, comment on pens and browse featured or trending work, using collections to group related pens together for reference or for teaching specific techniques.