Use width and height as intrinsic aspect ratio for img elements

When the current request is not available, or it doesn't have an intrinsic
ratio (such as some SVG content).

This specifies the proposal in
WICG/intrinsicsize-attribute#16, which is implemented
in both Firefox and Chromium behind a flag.

Tests for this change are at:

https://github.com/web-platform-tests/wpt/blob/a57ec1432f22ac42e8e219a32e2abd7c0baa5b09/html/rendering/replaced-elements/attributes-for-embedded-content-and-images/img-aspect-ratio.tentative.html

source

@@ -3638,6 +3638,11 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://drafts.csswg.org/css-color/#transparent-black">transparent black</dfn></li>
     </ul>
 
+    <p>The term <dfn
+    data-x-href="https://drafts.csswg.org/css-images/#intrinsic-aspect-ratio">intrinsic aspect
+    ratio</dfn> is used as defined in <cite>CSS Image Values and Replaced Content</cite> to define
+    the sizing of replaced content. <ref spec=CSSIMAGES></p>
+
     <p>The term <dfn data-x-href="https://drafts.csswg.org/css-images-4/#paint-source">paint
     source</dfn> is used as defined in <cite>CSS Image Values and Replaced Content</cite>
     to define the interaction of certain HTML elements with the CSS 'element()'
@@ -115556,6 +115561,13 @@ input[type=image i][align=bottom i], object[align=bottom i] {
   property">map to the dimension properties</span> <span>'width'</span> and <span>'height'</span> on
   the element respectively.</p>
 
+  <p>Additionally, for <code>img</code> elements for which their <span>current request</span> is not
+  <span data-x="img-available">available</span> or does not have an intrinsic ratio, their
+  <span>intrinsic aspect ratio</span> is computed as the ratio between the <code
+  data-x="attr-dim-width">width</code> and <code data-x="attr-dim-height">height</code> attributes,
+  if both are present, non-zero, and not a percentage when parsed using the <span>rules for parsing
+  dimension values</span>.</p>
+
   </div>