CodePen

codepen.io

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An online code editor and social platform for front-end developers to build, share, and discover HTML, CSS, and JavaScript snippets in an instant live-preview environment.

Why it is useful

The fastest way to prototype a UI idea or test a browser API — no setup, no bundler configuration, just code and a live preview. The community Pens are an excellent resource for understanding how specific CSS effects or JavaScript patterns are implemented, with fully editable source. Widely used for interview take-homes and sharing reproducible browser bugs.