ement` has
internal state that is not properly reset at the end of each test.

It would be trivial in the above example to simply reset `someElement.foo` to
the expected default value of `undefined` in an `afterEach` hook. However, for
non-contrived test suites that target complex elements, this can result in a
large quantity of ever-growing set-up and tear-down boilerplate.

@pseudoElement test-fixture
 -->

<script>

(function () {
  var TestFixturePrototype = Object.create(HTMLElement.prototype);
  var T