diff --git a/source b/source
index d117fe5a9d7..041d6b26830 100644
--- a/source
+++ b/source
@@ -3641,6 +3641,11 @@ 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()'
@@ -115559,6 +115564,22 @@ 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>The <span>intrinsic aspect ratio</span> for an <code>img</code> element <var>img</var> is
+  computed as follows:</p>
+
+  <ol>
+   <li><p>If <var>img</var>'s <span>current request</span> is <span
+   data-x="img-available">available</span> and has an <span>intrinsic aspect ratio</span>, then
+   use that intrinsic aspect ratio.</p></li>
+
+   <li><p>If <var>img</var>'s <code data-x="attr-dim-width">width</code> and <code
+   data-x="attr-dim-height">height</code> attribute values, when parsed using the <span>rules for
+   parsing dimension values</span>, are both not an error, not a percentage, and non-zero, then use
+   the ratio resulting from dividing the <code data-x="attr-dim-width">width</code> attribute value
+   by the <code data-x="attr-dim-height">height</code> attribute value.</p></li>
+
+   <li><p>Otherwise, <var>img</var> has no <span>intrinsic aspect ratio</span>.</p></li>
+   </ol>
   </div>