We call them Pens. Write HTML, CSS, and JS and get a real-time preview. You can use CodePen to:
- Show off the amazing things you build.
- Learn new things by tinkering.
- Build reduced test cases to report bugs and get help on tricky problems.
- Try out new technologies for the first time.
- Build components to later use elsewhere.
The possibilities are endless. Everything you do is saved on CodePen, so the more you use it the more useful it become
A front end web development playground.
Source: CodePen – Front End Developer Playground & Code Editor in the Browser