shadow-dom

This repo aims to provide a polyfill for Shadow DOM v1 that is as spec-complete and correct as possible.

What you will get if you use it right now:

• Shadow DOM polyfill
  • Polyfills Shadow DOM APIs like Element.prototype.attachShadow and Event.prototype.composedPath()
  • Polyfills Event and CustomEvent constructors to allow for composed events
  • Augments DOM querying/traversal/manipulation APIs to account for Shadow DOM
  • Augments MutationObserver to account for Shadow DOM
• Custom Elements polyfill
  • Built in/around the Shadow DOM polyfill
  • Shims HTMLElement and kin for browsers that have native support so transpiled and ES5-style classes can work (i.e. via var self = HTMLElement.call(this))

Please see the Caveats section below for details about certain APIs that are not polyfilled.

Browser Compatibility

To help with testing for cross-browser support, this project is receiving tremendous help from BrowserStack.

Browser	Version	Notes
Chrome	43	Chrome natively supports Shadow DOM since version 53 and Custom Elements since version 54.
Safari	9	Safari natively supports Shadow DOM since version 10.
Firefox	34
Edge	13	Edge 12 may be supported but it has not been verified.
Internet Explorer	11	IE 11 requires you to bring your own Promise polyfill if you want to use `customElements.whenDefined('my-element')`. IE 10 is not officially supported as it has some issues with transpiled ES6 custom element classes and requires you to bring your own Object.setPrototypeOf polyfill.

Usage

• Include dist/shadow-dom.js or dist/shadow-dom.min.js before any other scripts. It wouldn't be wise to use the async attribute for the <script>. Use the defer attribute with caution.
• If you want to force the Shadow DOM polyfill to be used in browsers with native support for some reason, set window.forceShadowDomPolyfill = true before the script is included.
• If you want to force the Custom Elements polyfill to be used in browsers with native support for some reason, set window.forceCustomElementsPolyfill = true before the script is included.

Caveats

• Benchmarks

  There are no benchmarks yet. Correctness comes first, which means benchmarks come after test-verified functionality.

• MutationObserver and garbage collection

  MutationObserver is polyfilled to account for shadow boundaries, but Javascript does not provide any sort of weak reference, so you must be sure to disconnect() your MutationObserver instances whenever it is appropriate to do so.

• <slot> and the display style property

  Browsers with native Shadow DOM treat <slot> elements as display: contents by default which simply cannot be properly polyfilled (think display: flex, display: table, etc.) With that in mind, you may want to adopt a practice of explicitly specifying display properties for <slot> elements.

• [CEReactions] and reflected content attributes

  Object properties that are said to reflect a content attribute will not cause attributeChangedCallback to be invoked (for example, setting className from script will not invoke the attributeChangedCallback for the class attribute.) This is because the [CEReactions] behavior for setters cannot be reasonably polyfilled for in Safari 9 and this project aims to provide a consistent level of support for its supported browsers.

• CSS Selectors

  No attempt is made to polyfill any selectors in CSS, like ::slotted, :host, or :host-context. These are also not polyfilled for APIs like matches, querySelector, and so forth.

• Obscure or low-priority DOM APIs

  APIs like Range, NodeIterator, and TreeWalker are not addressed. They may be in the future but they are currently very low-priority.

License

This project is licensed under the MIT license. See the LICENSE file.