diff --git a/index b/index
index d98b2bb665f..34a23696d57 100644
--- a/index
+++ b/index
@@ -12012,7 +12012,13 @@ brighter. A &lt;b>rat&lt;/b> scurries past the corner wall.&lt;/p></pre>
 
   <p><strong>Authoring requirements</strong>: The <code title=attr-img-src><a
    href="#src">src</a></code> attribute must be present, and must contain a
-   URI (or IRI). The requirements for the <code title=attr-img-alt><a
+   URI (or IRI).
+
+  <p class=big-issue>Should we restrict the URI to pointing to an image?
+   What's an image? Is PDF an image? (Safari supports PDFs in &lt;img>
+   elements.) How about SVG? (Opera supports those). WMFs? XPMs? HTML?
+
+  <p>The requirements for the <code title=attr-img-alt><a
    href="#alt">alt</a></code> attribute depend on what the image is intended
    to represent:
 
diff --git a/source b/source
index efca379a62c..d520a9aba83 100644
--- a/source
+++ b/source
@@ -9984,9 +9984,15 @@ brighter. A &lt;b>rat&lt;/b> scurries past the corner wall.&lt;/p></pre>
 
   <p><strong>Authoring requirements</strong>: The <code
   title="attr-img-src">src</code> attribute must be present, and must
-  contain a URI (or IRI). The requirements for the <code
-  title="attr-img-alt">alt</code> attribute depend on what the image
-  is intended to represent:</p>
+  contain a URI (or IRI).</p>
+
+  <p class="big-issue">Should we restrict the URI to pointing to an
+  image? What's an image? Is PDF an image? (Safari supports PDFs in
+  &lt;img> elements.) How about SVG? (Opera supports those). WMFs?
+  XPMs? HTML?</p>
+
+  <p>The requirements for the <code title="attr-img-alt">alt</code>
+  attribute depend on what the image is intended to represent:</p>
 
   <dl>