or: blue;
}

:only-child {
  background-color: blue;
}

.foo:only-child {
  background-color: blue;
}
</style>

<script>
promise_test(() => {
  assert_implements(document.startViewTransition, "Missing document.startViewTransition");
  return new Promise(async (resolve, reject) => {
    let transition = document.startViewTransition();
    transition.ready.then(() => {
      let style = getComputedStyle(
        document.documentElement, "::view-transition");
      if (style.backgroundColor == "rgb(255, 0, 0)