whenDefined(name)

<nav id="menu-container">
  <div class="menu-placeholder">Loading…</div>
  <nav-menu>
    <menu-item>Item 1</menu-item>
    <menu-item>Item 2</menu-item>
    …
    <menu-item>Item N</menu-item>
  </nav-menu>
</nav>

const container = document.getElementById("menu-container");
const placeholder = container.querySelector(".menu-placeholder");
// Fetch all the children of menu that are not yet defined.
const undefinedElements = container.querySelectorAll(":not(:defined)");

async function removePlaceholder() {
  // Filter the elements down to unique localNames
  const tags = new Set(
    [...undefinedElements].map((button) => button.localName),
  );
  const promises = [...tags].map((tag) => customElements.whenDefined(tag));

  // Wait for all the children to be upgraded
  await Promise.all(promises);
  // then remove the placeholder
  container.removeChild(placeholder);
}

removePlaceholder();