How can I use the :first-child selector w/ shadow DOM's <content> element?

Question

I'd like to extract the first paragraph from a custom element's content (as a summary). I would expect the following to work:

<polymer-element name="x-thing" noscript>
  <template>
    <segment class="summary">
      <content select="p:first-child"></content>
    </segment>
    <segment>
      <content></content>
    </segment>
  </template>
</polymer-element>

Unfortunately, the <content select="p:first-child"> doesn't appear to match anything :(

Here's a corresponding codepen.

Answer 1

Only :not() is supported in the Shadow DOM spec: http://w3c.github.io/webcomponents/spec/shadow/#dfn-matching-criteria

Relevant bug: http://crbug.com/343332