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NPM GitHub Workflow Status (master) Codacy code quality Codacy branch coverage License: MIT jsDelivr Sponsor this project

A ponyfill that provides client-side support for CSS custom properties (aka "CSS variables") in legacy and modern browsers.


  • Client-side transformation of CSS custom properties to static values
  • Live updates of runtime values in both modern and legacy browsers
  • Transforms <link>, <style>, and @import CSS
  • Transforms relative url() paths to absolute URLs
  • Supports chained and nested var() functions
  • Supports var() function fallback values
  • Supports web components / shadow DOM CSS
  • Watch mode auto-updates on <link> and <style> changes
  • UMD and ES6 module available
  • TypeScript definitions included
  • Lightweight (6k min+gzip) and dependency-free


  • Custom property declaration support is limited to :root and :host rulesets
  • The use of var() is limited to property values (per W3C specification)
  • CSS changes made using CSSOM APIs are not supported (see #19, #23, #77, #154).

Browser Support

Chrome 19+
Edge 12+
Firefox 6+
IE 9+
Safari 6+

Usage & Options

See the documentation site for details.


A sponsorship is more than just a way to show appreciation for the open-source authors and projects we rely on; it can be the spark that ignites the next big idea, the inspiration to create something new, and the motivation to share so that others may benefit.

If you benefit from this project, please consider lending your support and encouraging future efforts by becoming a sponsor.

Thank you! 🙏🏻

Contact & Support

  • Follow 👨🏻‍💻 @jhildenbiddle on Twitter and GitHub for announcements
  • Create a 💬 GitHub issue for bug reports, feature requests, or questions
  • Add a ⭐️ star on GitHub and 🐦 tweet to promote the project
  • Become a 💖 sponsor to support the project and future efforts


This project is licensed under the MIT License. See the MIT LICENSE for details.

Copyright (c) John Hildenbiddle (@jhildenbiddle)