User JavaScript and CSS Autor: tenRabbits

Write your own JavaScript and CSS for any website: hide what gets in the way, add a missing button, fix a layout. A rule pairs your code with the sites it runs on, and the extension applies it every time you open them.

Editor
- Side-by-side JavaScript and CSS editors with syntax highlighting, Prettier formatting and 13 themes
- SCSS and Sass out of the box; the preprocessor can be turned off for a rule
- Styles update on open tabs as you save — no reload needed
- The last 10 versions of each rule are kept, so a bad edit is one click away from undo

Rules and sites
- Several URL patterns per rule, with exclusions: !https://example.com/admin/*
- Styles can apply inside nested frames and open shadow DOM — per rule
- Any rule can serve as a template for new ones or as a module for other rules

Libraries
- Add jQuery or any script and style by URL; files are stored locally and update daily, weekly or on demand

Backup
- Export and import in JSON — pick which rules to import
- Backup to the browser's sync storage

In Firefox
- The Firefox version is new and marked experimental. If something does not work, write to the support email on this page
- JavaScript runs through the Firefox user scripts API: grant this permission with the button on the settings page. Styles work without it
- Rules need access to websites. If it is missing, the settings page and the toolbar popup offer to grant it

AI agent tools (experimental). Let an agent on your computer — Claude Code, Cursor, Codex, VS Code and other MCP clients — manage your rules. Off by default; enable in Settings → MCP. The connection is local only: the extension connects to ws://127.0.0.1:8767, served by the user-js-css-mcp server your agent starts. Nothing leaves your machine; both sides verify a shared secret. The agent can read and change your rules; the secret is never included in exports. Turn it off with the same switch. Server: https://www.npmjs.com/package/user-js-css-mcp

Privacy. No ads, no analytics, no data collection. Your code stays in your browser; the extension only downloads the libraries you add.

The interface is in English and Russian. A personal, non-commercial project.

Documentation: https://tenrabbits.github.io/user-js-css-docs/