Fix details inside drop-down being rendered as dropdown

[mwargan]

Given a dropdown that contains an accordion within it, the accordion renders as a dropdown because the summary selector also selects the children of a dropdown element. This fixes that so it renders as expected.

<details class="dropdown">
   <summary> Select a phase of matter... </summary>
   <ul>
      <li>
         <details>
            <summary> This is a dropdown option </summary>
            <ul>
               <li><label><input type="radio" name="phase" value="gas"> Gas </label></li>
               <li><label><input type="radio" name="phase" value="plasma"> Plasma </label></li>
            </ul>
         </details>
      </li>
   </ul>
</details>

Note this fix also more closely conforms with the HTML spec https://html.spec.whatwg.org/multipage/interactive-elements.html#the-summary-element as the summary element should be used in the context "As the first child of a details element."