Specify navigator.appVersion to match reality

Closes #7773.

Author: hsivonen

source

@@ -98800,10 +98800,35 @@ interface <dfn interface>Navigator</dfn> {
    <dt><dfn attribute for="NavigatorID"><code data-x="dom-navigator-appName">appName</code></dfn></dt>
    <dd><p>Must return the string "<code data-x="">Netscape</code>".</p></dd>
 
-   <dt><dfn attribute for="NavigatorID"><code data-x="dom-navigator-appVersion">appVersion</code></dfn></dt>
-   <dd><p>Must return either the string "<code data-x="">4.0</code>" or a string representing the
-   version of the browser in detail, e.g. "<code data-x="">1.0 (VMS; en-US)
-   Mellblomenator/9000</code>".</p></dd>
+   <dt><dfn attribute for="NavigatorID"><code
+   data-x="dom-navigator-appVersion">appVersion</code></dfn></dt>
+   <dd>
+    <p>Must return the appropriate string that starts with "<code data-x="">5.0 (</code>", as
+    follows:</p>
+
+    <p>Let <var>trail</var> be the substring of <span data-x="default-user-agent-value">default
+    `<code>User-Agent</code>` value</span> that follows the "<code data-x="">Mozilla/</code>"
+    prefix.</p>
+    <!-- Fetch doesn't actually require the value to start with "Mozilla/5.0 (". -->
+
+    <dl class="switch"> <dt>If the <span data-x="concept-navigator-compatibility-mode">navigator
+    compatibility mode</span> is <i>Chrome</i> or <i>WebKit</i></dt>
+    <dd><p>Return <var>trail</var>.</p></dd>
+
+     <dt>If the <span data-x="concept-navigator-compatibility-mode">navigator compatibility
+     mode</span> is <i>Gecko</i></dt>
+     <dd>
+      <p>If <var>trail</var> starts with "<code data-x="">5.0 (Windows</code>", then return "<code
+      data-x="">5.0 (Windows)</code>".</p>
+
+      <p>Otherwise, return the prefix of <var>trail</var> up to but not including the first U+003B
+      (;), concatenated with the character U+0029 RIGHT PARENTHESIS. For example, "<code
+      data-x="">5.0 (Macintosh)</code>", "<code data-x="">5.0 (Android 10)</code>", or "<code
+      data-x="">5.0 (X11)</code>".</p> <!-- Unclear if Gecko mode returning a shorter string that
+      doesn't contain the whole trail is needed for Web compat. -->
+     </dd>
+    </dl>
+   </dd>
 
    <!-- buildID: Mozilla only -->