Every online code editor wants an account before you run line one. For a quick test — check a regex, format some JSON, try a Python snippet — that's backwards. You shouldn't need an account, a server round-trip, or to upload your code anywhere just to answer a small question.

I use a free set of tools that all run client-side, in your own browser. Here's what they cover, and when each one saves you time.

1. Test HTML, CSS and JavaScript instantly

Open the playground, type, and the live preview updates about a second after you stop typing. No Run button, no server. When you're happy, download your work as a single HTML file — it survives a closed tab because autosave lives in your own browser, not an account.

2. Format JSON — and find the exact error

Paste messy JSON into the formatter. If it's broken, it tells you the exact line and column of the error instead of a vague "invalid JSON". The validator does the same check on its own when you just want a yes/no answer.

3. Run Python and SQL without installing anything

The Python playground runs CPython in your browser through WebAssembly (Pyodide), and the SQL playground runs SQLite the same way (sql.js). Nothing is sent to a server. Handy for testing a loop, a query, or a function before you paste it into a real project.

4. Test a regex before it breaks your code

Type your pattern and your test text side by side. Matches highlight live, so you can see what your pattern actually catches — not what you hoped it catches.

5. The rest of the toolbox

  • YAML Formatter — clean up config files
  • Markdown Editor — write and preview docs
  • Code Diff Checker — see exactly what changed between two versions
  • Code to Image — turn a snippet into a clean screenshot for docs or social posts
  • Assembly Compiler Online — try low-level code in the browser

That's 14 tools in total: HTML Editor, CSS Editor, JavaScript Playground, JSON Formatter & Validator, JSON Validator, Regex Tester, Markdown Editor, SQL Playground, Python Playground, Online Python Compiler, YAML Formatter, Code Diff Checker, Code to Image, and Assembly Compiler Online.

When to use this, and when not to

Use zero-setup tools for quick, private scratch work — testing, learning, debugging a snippet. CodePen is still the better choice when you want a public portfolio and a community around your work. For a private scratchpad, no-signup wins: you open it, do the job, and leave, with your code never leaving your device.

The set I use is CodeLive Studio — free, 14 tools, no signup: https://codelivestudio.cyou