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

<script src="./index.js"></script>
<script>
  function __ssr () {
    const script = document.currentScript;
    const host = script.previousElementSibling;
    const fakeShadowRoot = host.firstElementChild;
    const slots = host.getElementsByTagName('slot');
    const move = (from, to) => { while (from.firstChild) to.appendChild(from.firstChild) };

    // At each Shadow Root, we only care about its slots, not composed slots,
    // therefore we need to move the children of top level slots, but no others
    // Also can't 'move' in loop as that will mutate the DOM and ruin the 
    // 'contains' checks for subsequent slots.
    const topLevelSlots = (() => {
      let top = [],
          ref;

      for (let i = 0, k = slots.length; i < k; i++) {
        const slot = slots[i];

        // Ref is last known top level slot, if current slot is contained by it,
        // then that slot is nested and can be ignored
        if (!(ref && ref.contains(slot))) {
          top.push(slot);
          ref = slot;
        }        
      }

      return top;
    })();

    topLevelSlots.forEach(slot => move(slot, host));

    // // 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' });
    move(fakeShadowRoot, realShadowRoot);
    host.removeChild(fakeShadowRoot);
  }
</script>
<script>
  test(a => {
    const frag = document.createDocumentFragment();
    const div = document.createElement('div');
    const script = document.createElement('script');

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

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

    return frag;
  });
</script>