::slotted() should full support complex selector!!

[minzojian]

i wrote a demo here

for good to SEO,we need support a clear dom structure in html, eg. there is a navbar component,and i add some links inside,and planning to inject them into a navbar component. (even it is not for good to SEO, as a common reuseable component, elements should be inject outside freely)

  <my-navbar>
    <ul>
      <li>link1</li>
      <li>link2</li>
      <li>link3</li>
      </ul>
  </my-navbar>

however when i try to add some style for the < li > tag inside my navbar, it just not work
i found a explanation that slotted is just use simple selector
i wonder why the ::slotted selector can't support the complex selector (or child selector)?
::slotted() should full support complex selector
it just lame if not support

just copied from WICG/webcomponents#745