This is speculation: The reason Google is moving forward quickly with Shadow DOM implementation is that this polyfill is dog slow. Think about what it takes to make DOM invisible in the browser in a polyfill- overriding every DOM method to filter out shadowed nodes. I've heard reports of 10x slowdowns on DOM operations with this polyfill in place. The kicker? Web Components (specifically Custom Elements) lose almost all their encapsulation without Shadow DOM. Those of us who are betting heavy on Web Components as The Future™ are pretty anxious about this.