Online Editors

An online editor (a "playground") runs in a browser tab: you type HTML, CSS and JavaScript into panes and a live preview updates beside them. Nothing to install, and every piece of work gets a URL, so playgrounds are the standard way to ask for help on Stack Overflow, file a reproducible bug report, try a library, or embed a working demo in a blog post. Framework teams use them for official tutorials: Svelte 36,701 , Angular 42,088 and Nuxt 37,147 run theirs on StackBlitz 38,726 's WebContainers.

JSFiddle 11,877 's four-pane editor: HTML, CSS, JavaScript and Result

Online editors compared
Playground	Where code runs	Best for	Free tier
CodePen	Sandboxed iframe in your browser	Front-end demos, embeds, inspiration	Public Pens
JSFiddle	Sandboxed iframe in your browser	Minimal bug reproductions	Yes, with ads
StackBlitz	Node.js 2,131 in your browser (WebContainers)	npm 2,036 projects, Vite 25,978 , Angular	Public projects
CodeSandbox 24,655	Cloud VMs (Devboxes) and browser sandboxes	Full-stack apps, pull request previews	About 40 VM hours a month
Replit 3,526	Cloud containers	Many languages, hosting, AI agent	Starter; Core $20/month

CodePen and JSFiddle are the simplest: your panes are combined into one document and run in a sandboxed iframe on a separate domain (CodePen uses cdpn.io), so a demo cannot read the editor site's cookies. Add npm packages as CDN URLs. JSFiddle PRO (€8 a month) removes ads and adds private fiddles.

StackBlitz goes further: WebContainers, a WebAssembly-based micro operating system, run npm install and npm run dev inside the tab, with no remote server, and cache node_modules in IndexedDB. They need SharedArrayBuffer and cross-origin isolation, so Chromium 4,389 browsers get full support while Firefox 555 and Safari 16.4 10 + are in beta. Open any GitHub 29 repository by rewriting its address as https://stackblitz.com/fork/github/USER/REPO 38,726 . The same company built the AI app generator bolt.new on this engine.

CodeSandbox (acquired by Together AI in December 2024) and Replit run your project on real cloud machines, which can host databases and back-end languages but consume usage quotas. Glitch, once popular for Node.js hobby apps, shut down on July 8, 2025, taking its hosted projects offline: keep your own copy of anything important in Git 1,932 .

CodePen can also create a Pen for your readers. POST a JSON description to its Prefill API, and every "Edit on CodePen" button opens a new Pen with your code already loaded.

Opening a prefilled CodePen from any web pageHTMLLive

<form action="https://codepen.io/pen/define" method="POST" target="_blank">
  <input type="hidden" name="data" value='{
    "title": "Grid centering",
    "html": "<div class=\"box\">Centered</div>",
    "css": "body{display:grid;place-items:center;height:100vh} .box{background:#e3f2fd}",
    "editors": "110"
  }'>
  <button>Edit on CodePen</button>
</form>

editors is a three-digit HTML/CSS/JS mask: 110 opens the HTML and CSS panes and collapses the JavaScript pane.

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