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Learn the tech

Basic usage

var el = document.createElement('div');
var shadow = el.createShadowRoot();
shadow.innerHTML = '<content select="h1"></content>';

Shadow DOM subtrees

Shadow DOM allows a single node to express three subtrees: light DOM, shadow DOM, and composed DOM.

A component user supplies the light DOM; the node has a (hidden) shadow DOM; and the composed DOM is what is actually rendered in the browser. At render time, the light DOM is merged with the shadow DOM to produce the composed DOM. For example:

Light DOM

  <!-- everything in here is my-custom-element's light DOM -->
  <q>Hello World</q>

Shadow DOM

<!-- shadow-root is attached to my-custom-element, but is not a child -->
  <!-- everything in here is my-custom-element's shadow DOM -->
  <span>People say: <content></content></span>

Composed (rendered) DOM

<!-- rendered DOM -->
  <span>People say: <q>Hello World</q></span>

The following is true about this example:

  • The light DOM that belongs to <my-custom-element> is visible to the user as its normal subtree. It can expressed by .childNodes, .children, .innerHTML or any other property or method that gives you information about a node's subtree.
  • Nodes in light DOM or shadow DOM express parent and sibling relationships that match their respective tree structures; the relationships that exist in the rendered tree are not expressed anywhere in DOM.

So, while in the final rendered tree <span> is a child of <my-custom-element> and the parent of <q>, interrogating those nodes will tell you that the <span> is a child of <shadow-root> and <q> is a child of <my-custom-element>, and that those two nodes are unrelated.

In this way, the user can manipulate light DOM or shadow DOM directly as regular DOM subtrees, and let the system take care of keeping the render tree synchronized.

Polyfill details

A polyfill to provide Shadow DOM functionality in browsers that don't support it natively. This section explains how a proper (native) implementation differs from our polyfill implementation.

Logical DOM

The light DOM and the shadow DOM is referred to as the logical DOM. This is the DOM that the developer interacts with. The composed DOM is what the browser sees and uses to render the pixels on the screen.


The polyfill is implemented using wrappers. A wrapper wraps the native DOM node in a wrapper node. The wrapper node looks and behaves identical to the native node (minus bugs and known limitations). For example:

var div = document.createElement('div');
div.innerHTML = '<b>Hello world</b>';
assert(div.firstChild instanceof HTMLElement);

But div is actually a wrapper of the element that the browser normally gives you. This wrapper just happen to have the same interface as the browser provided element.

It has an innerHTML setter that works just like the native innerHTML but it instead of working on the composed tree it works on the local DOM. When you change the logical DOM tree like this it might cause the composed tree to need to be re-rendered. This does not happen immediately, but it is scheduled to happen later as needed.

The wrapper node also have a firstChild getter which once again works on the logical DOM.

instanceof still works because we have replaced the global HTMLElement constructor with our custom one.

More Logical DOM

The wrappers.Node object keeps track of the logical (light as well as shadow, but not composed) DOM. Internally it has has the 5 fundamental Node pointers, parentNode, firstChild, lastChild, nextSibling and previousSibling. When the DOM tree is manipulated these pointers are updated to always represent the logical tree. When the shadow DOM renderer needs to render the visual tree, these internal pointers are updated as needed.

Wrap all the objects!

The intent is to wrap all the DOM objects that interact with the DOM tree. For this polyfill to be completely transparent we need to wrap a lot of APIs. Any method, accessor or constructor that takes or returns a Node or an object that indirectly touches a node needs to be wrapped. As you can imagine there are a lot of these. At the moment we have done the most common ones but there are sure to be missing ones as soon as you try to use this with your code.

wrap and unwrap

There are bound to be cases where we haven't done the wrapping for you. In those cases you can use wrap to create a wrapper of a native object, or unwrap to get the underlying native object from a wrapper. These two functions are available on the ShadowDOMPolyfill object.

Event Retargetting

An important aspect of the shadow DOM is that events are retargetted to never expose the shadow DOM to the light DOM. For example.

var div = document.createElement('div');
div.innerHTML = 'Click me';
var sr = div.createShadowRoot();
sr.innerHTML = '<b><content></content></b>';

If the user clicks on the div the real target of the click event is the <b> element. But that element is not visible in the light DOM so the target is therefore retargetted to the div element itself. However, if there is an event listener on the <content>, <b> or the shadow root, the target should be visible to the event listener.

Similar issues occur with relatedTarget in mouseover and mouseout events.

To support this kind of behavior the event dispatching in the browser has to be reimplemented by the polyfill.

Known issues

  • CSS encapsulation is not implemented.
  • Object.prototype.toString does not return the same string as for native objects.
  • No live NodeLists. All node lists are snapshotted upon read.
  • document, window, document.body, document.head and others are non configurable and cannot be overridden. We are trying to make these work as seamlessly as possible but there will doubtlessly be cases where there will be problems; for those cases you can use wrap and unwrap to get unblocked.
  • onclick and other on* attribute event handler do not wrap the event object as needed.
  • Cross window/frame access is not implemented.


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