Example:

var Modal = document.registerElement('x-modal', {
  prototype: Object.create(HTMLElement, {/* modal generic prototype */})
})
var WebcamModal = document.registerElement('x-webcam', {
  extends: 'x-modal',
  prototype: Object.create(Modal, {
    /* webcam modal specific prototype */
    show: {
        value: function() {
          Modal.prototype.show.call(this)
          // run webcam
        }
    }
  })
})

<x-modal></x-modal>
<x-modal is="webcam"></x-modal>

And will be cool if will be possible to have a non-dashed name of extend name (is="webcam" instead is="x-webcam").