> As soon as they try to build any kind of container element they hit big problems.

That's not true. Web components that render nothing will contain only their children as nodes, that's good enough for a good amount of container use cases.

So you can have something like:

  <uix-modal>
    <uix-button icon="wifi"></uix-button>
    <dialog>
      <div class="flex flex-col gap-4 p-4 w-[640px]">
       ...
      </div>
    </dialog> 
  <uix-modal>