Add more to the Note about why the behavior is the way it is

Author: mfreed7

source

@@ -79337,6 +79337,14 @@ interface <dfn interface>VisibilityStateEntry</dfn> : <span>PerformanceEntry</sp
 &lt;div style="interactivity:inert">
   &lt;dialog>&lt;/dialog>
 &lt;/div></code></pre>
+
+    <p>The reason that HTML "overrides" the <span data-x="css-interactivity">'interactivity'</span>
+    property in the case of modal dialogs is that accessibility technology requires modal dialogs to
+    be truly modal. Content outside of an open modal dialog should always be inert. For this reason,
+    the spec is designed in such a way that developers are unable to override modal dialog <span
+    data-x="css-interactivity">'interactivity'</span>, while still being able to override other
+    developer-controlled forms of interactivity such as the <code data-x="attr-inert">inert</code>
+    attribute.</p>
   </div>
 
   <h4>The <dfn data-x="attr-inert"><code>inert</code></dfn> attribute</h4>