CustomElementRegistry.whenDefined()

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(){
  const promises = [...undefinedElements].map(
    button => customElements.whenDefined(button.localName)
  );

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

removePlaceholder();