A cross-browser polyfill for the Custom Elements portion of the W3C Web Components specification.

Usage no npm install needed!

<script type="module">
  import customElements from '';


Custom Elements

Custom elements are a part of the W3C Web Components specification (see spec). They allow you to define and register new HTML tags/elements in your documents. You can then use these tags as regular HTML.

This library polyfills the Custom Elements API on browsers today. It is a barebones fork of the X-Tag core library from Mozilla (see website). X-Tags in turn uses the Polymer polyfills from Google (see website).

The aim of this fork is to provide a stripped down version of the above polyfills, with zero additional weight.

What’s included?

There are two source files in the repository:

  • CustomElements.js, which polyfills the W3C Web Components Custom Elements API.
  • MutationObserver.js, which polyfills the MutationObserver API. This is needed for the CustomElements polyfill and additionally polyfills the WeakMap API.

Each of these files are minified for distributions (see the dist directory).

How do I use it?

Including CustomElements.js (or CustomElements.min.js) in your source will polyfill the Custom Elements API. For browsers that already support CustomMutations, it is not necessary to include CustomMutations.js (CustomMutations.min.js), although it will do no harm if you do (apart from possibly polyfilling the WeakMap API unecessarily).

Sample code is located in the demo directory. An introduction to the Custom Elements API (with code examples) is available on the HTML5 Rocks website from Google.


This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, You can obtain one at

Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

Thanks to Arron Schaar and Daniel Buchner of Mozilla for their help.