@skatejs/ssr/perf/use-script.html

<script src="./index.js"></script>
<script>
  function __ssr () {
    const script = document.currentScript;
    const host = script.previousElementSibling;
    const fakeShadowRoot = host.firstElementChild;

    // Removing the script speeds up rendering by a few hundred ms when
    // not optimised and only a few when optimised.
    script.parentNode.removeChild(script);

    // The fastest overall method is to simply use innerHTML. This seems to be
    // faster when not optimised (by about 50%) but is slightly slower when
    // optimised compared to traversing the fake shadow root and appending each
    // element to the real one:
    //
    // const realShadowRoot = host.attachShadow({ mode: 'open' });
    // while (fakeShadowRoot.firstChild) realShadowRoot.appendChild(fakeShadowRoot.firstChild);
    host.attachShadow({ mode: 'open' }).innerHTML = fakeShadowRoot.innerHTML;
  }
</script>
<script>
  test(a => {
    const frag = document.createDocumentFragment();
    const div = document.createElement('div');
    const script = document.createElement('script');

    div.innerHTML = `<shadow-root><strong><slot></slot></strong></shadow-root>${a}`;
    script.innerHTML = `__ssr();`;

    frag.appendChild(div);
    frag.appendChild(script);

    return frag;
  });
</script>