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).

Releases

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

webcomponents-lite.js includes all of the polyfills.
webcomponents-loader.js is a custom loader that dynamically load a minified polyfill bundle, using feature detection. The bundles that can be loaded are:
- webcomponents-hi -- HTML Imports (needed by Safari Tech Preview)
- webcomponents-hi-ce -- HTML Imports and Custom Elements (needed by Safari 10)
- webcomponents-hi-ce-sd -- HTML Imports, Custom Elements and Shady DOM/CSS (needed by Safari 9, Firefox, Edge)
- webcomponents-lite -- HTML Imports, Custom Elements, Shady DOM/CSS and generic platform polyfills (such as URL, Template, ES6 Promise, Constructable events, etc.) (needed by Internet Explorer 11)

Browser Support

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

Polyfill	IE11+	Chrome*	Firefox*	Safari 9+*	Chrome Android*	Mobile Safari*
Custom Elements	✓	✓	✓	✓	✓	✓
HTML Imports	✓	✓	✓	✓	✓	✓
Shady CSS/DOM	✓	✓	✓	✓	✓	✓

*Indicates the current version of the browser

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 npm 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
bower install
# build
gulp build

The builds will be placed into the root directory.

Contribute

See the contributing guide

License

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

Helper utilities

`WebComponentsReady`

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-lite.js polyfill (as well as the smaller bundles and the loader) 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, 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

Custom element's constructor property is unreliable
Contenteditable elements do not trigger MutationObserver
ShadyCSS: :host(.zot:not(.bar:nth-child(2))) doesn't work

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

ShadyCSS: :host(.zot:not(.bar:nth-child(2))) doesn't work

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