This makes a number of related changes to specify the rendering of the
<details> element:

* It specifies the structure of the user agent shadow tree.  This
  appears largely interoperable between implementations with the
  exception of the style or link element for the default summary styles:
  Gecko uses a link element as the first child, Chromium uses a style
  element as the last child, and WebKit does not use a style element
  (see below).  This specifies a style or link as the first child.
* It specifies the existence of the default summary and the presence of
  UA dependent text inside of it.  This is present in all of Chromium,
  Gecko, and WebKit.
* It specifies the styles needed for the default summary.  These match
  Chromium and Gecko.  (These are not present in WebKit because WebKit's
  mechanism for styling the marker does not match the existing spec.)
* It removes the restriction that <details> is a block and cannot be
  changed.  This is prototyped in Chromium and Gecko.  This fixes whatwg#9830.
* It specifies that the summary element is display: block by default.
  This matches all of Chromium, Gecko, and WebKit.
* It specifies which element matches the ::details-content
  pseudo-element.  (TODO: This needs to be specified in CSS)  This is
  prototyped in Chromium.

See w3c/csswg-drafts#9879 and
w3c/csswg-drafts#9951 for more background.

…o be the first child.