Both editors are free, open-source, and work entirely in your browser.
ECodePen
By OpenSource
A full-featured in-browser IDE powered by CodeMirror. Syntax highlighting, multi-panel layout, and an instant preview pane. Perfect for experimenting with web UIs.
A minimal scratchpad with a live preview that updates as you type. No buttons, no overhead — just write HTML, CSS, and JS and watch it render instantly.