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A suite of polyfills supporting the Web Components specs:

Custom Elements: allows authors to define their own custom tags (spec).

HTML Imports: a way to include and reuse HTML documents via other HTML documents (spec).

Shadow DOM: provides encapsulation by hiding DOM subtrees under shadow roots (spec).

This also folds in polyfills for MutationObserver and WeakMap.


Pre-built (concatenated & minified) versions of the polyfills are maintained in the tagged versions of this repo. There are two variants:

webcomponents.js includes all of the polyfills.

webcomponents-lite.js includes all polyfills except for shadow DOM.

Browser Support

Our polyfills are intended to work in the latest versions of evergreen browsers. See below for our complete browser support matrix:

Polyfill IE10 IE11+ Chrome* Firefox* Safari 7+* Chrome Android* Mobile Safari*
Custom Elements ~
HTML Imports ~
Shadow DOM

*Indicates the current version of the browser

~Indicates support may be flaky. If using Custom Elements or HTML Imports with Shadow DOM, you will get the non-flaky Mutation Observer polyfill that Shadow DOM includes.

The polyfills may work in older browsers, however require additional polyfills (such as classList) to be used. We cannot guarantee support for browsers outside of our compatibility matrix.

Manually Building

If you wish to build the polyfills yourself, you’ll need node and gulp on your system:

  • install node.js using the instructions on their website
  • use npm to install gulp.js: npm install -g gulp

Now you are ready to build the polyfills with:

# install dependencies
npm install
# build
gulp build

The builds will be placed into the dist/ directory.


See the contributing guide


Everything in this repository is BSD style license unless otherwise specified.

Copyright (c) 2015 The Polymer Authors. All rights reserved.

Helper utilities


Under native HTML Imports, <script> tags in the main document block the loading of such imports. This is to ensure the imports have loaded and any registered elements in them have been upgraded.

The webcomponents.js and webcomponents-lite.js polyfills parse element definitions and handle their upgrade asynchronously. If prematurely fetching the element from the DOM before it has an opportunity to upgrade, you’ll be working with an HTMLUnknownElement.

For these situations (or when you need an approximate replacement for the Polymer 0.5 polymer-ready behavior), you can use the WebComponentsReady event as a signal before interacting with the element. The criteria for this event to fire is all Custom Elements with definitions registered by the time HTML Imports available at load time have loaded have upgraded.

window.addEventListener('WebComponentsReady', function(e) {
  // imports are loaded and elements have been registered
  console.log('Components are ready');

Known Issues

Limited CSS encapsulation

Under native Shadow DOM, CSS selectors cannot cross the shadow boundary. This means document level styles don’t apply to shadow roots, and styles defined within a shadow root don’t apply outside of that shadow root. Several selectors are provided to be able to deal with the shadow boundary.

The Shadow DOM polyfill can’t prevent document styles from leaking into shadow roots. It can, however, encapsulate styles within shadow roots to some extent. This behavior isn’t automatically emulated by the Shadow DOM polyfill, but it can be achieved by manually using the included ShadowCSS shim:

WebComponents.ShadowCSS.shimStyling( shadowRoot, scope );

… where shadowRoot is the shadow root of a DOM element, and scope is the name of the scope used to prefix the selectors. This removes all <style> elements from the shadow root, rewrites it rules using the given scope and reinserts the style as a document level stylesheet. Note that the :host and :host-context pseudo classes are also rewritten.

For a full explanation on the implementation and both the possibilities and the limitations of ShadowCSS please view the documentation in the ShadowCSS source.

Element wrapping / unwrapping limitations

The Shadow DOM polyfill is implemented by wrapping DOM elements whenever possible. It does this by wrapping methods like document.querySelector to return wrapped DOM elements. This has a few caveats:

  • Not everything can be wrapped. For example, elements like document, window, document.body, document.fullscreenElement and others are non-configurable and thus cannot be overridden.
  • Wrappers don’t support live NodeLists like HTMLElement.childNodes and HTMLFormElement.elements. All NodeLists are snapshotted upon read. See #217 for an explanation.

In order to work around these limitations the polyfill provides the ShadowDOMPolyfill.wrap and ShadowDOMPolyfill.unwrap methods to respectively wrap and unwrap DOM elements manually.

Custom element’s constructor property is unreliable

See #215 for background.

In Safari and IE, instances of Custom Elements have a constructor property of HTMLUnknownElementConstructor and HTMLUnknownElement, respectively. It’s unsafe to rely on this property for checking element types.

It’s worth noting that customElement.__proto__.__proto__.constructor is HTMLElementPrototype and that the prototype chain isn’t modified by the polyfills(onto ElementPrototype, etc.)

Contenteditable elements do not trigger MutationObserver

Using the MutationObserver polyfill, it isn’t possible to monitor mutations of an element marked contenteditable. See the mailing list

ShadowCSS: :host-context(…):host(…) doesn’t work

See #16 for background.

Under the shadow DOM polyfill, rules like:

:host-context(.foo):host(.bar) {...}

don’t work, despite working under native Shadow DOM. The solution is to use polyfill-next-selector like:

polyfill-next-selector { content: '.foo,'; }

ShadowCSS: :host(.zot:not(.bar:nth-child(2))) doesn’t work

ShadowCSS :host() rules can only have (at most) 1-level of nested parentheses in its argument selector under ShadowCSS. For example, :host(.zot) and :host(.zot:not(.bar)) both work, but :host(.zot:not(.bar:nth-child(2))) does not.

HTML imports: document.currentScript doesn’t work as expected

In native HTML Imports, document.currentScript.ownerDocument references the import document itself. In the polyfill use document._currentScript.ownerDocument (note the underscore).

execCommand and contenteditable isn’t supported under Shadow DOM

See #212

execCommand, and contenteditable aren’t supported under the ShadowDOM polyfill, with commands that insert or remove nodes being especially prone to failure.