Add ::details-content pseudo-element. (#10379)

This adds the ::details-content pseudo-element as resolved in:

1. #9879 (comment)

2. whatwg/html#10200 (comment) /
   #9951 (comment)

3. #9879 (comment)

and uses the definition added in #10083.

css-pseudo-4/Overview.bs

@@ -1310,6 +1310,33 @@ File Selector Button: the ''::file-selector-button'' pseudo-element</h3>
  <pre class="lang-css">::file-selector-button { border: 3px solid green }</pre>
  </div>
 
+<h3 id="details-content-pseudo">
+Expandable contents of details element: the ''::details-content'' pseudo-element</h3>
+
+ The <dfn>::details-content</dfn> pseudo-element targets
+ the additional information in a a <{details}> element
+ that can be expanded or collapsed.
+ It is a [=part-like pseudo-element=].
+
+ There is no restriction on which properties apply to the
+ ''::details-content'' pseudo-element.
+
+ <div class="example" id="details-content-example">
+ For example, the following example would
+ animate the opacity of the additional information
+ when the <{details}> element opens:
+
+ <pre class="lang-css">details::details-content {
+ opacity: 0;
+ transition: content-visibility 300ms allow-discrete step-end, opacity 300ms;
+}
+
+details[open]::details-content {
+ opacity: 1;
+ transition: content-visibility 300ms allow-discrete step-start, opacity 300ms;
+}</pre>
+ </div>
+
 <h2 id="interactions">
 Overlapping Pseudo-element Interactions</h2>