Convert user prompt types into a separate type

index.bs

@@ -2658,6 +2658,17 @@ browsingContext.ReadinessState = "none" / "interactive" / "complete"
 The <code>browsingContext.ReadinessState</code> type represents the stage of
 document loading at which a navigation command will return.
 
+#### The browsingContext.UserPrompt Type #### {#type-browsingContext-UserPrompt}
+
+[=Remote end definition=] and [=local end definition=]
+
+<pre class="cddl remote-cddl local-cddl">
+browsingContext.UserPromptType = "alert" / "confirm" / "prompt" / "beforeunload";
+</pre>
+
+The <code>browsingContext.UserPrompt</code> type represents the possible user
+prompt types.
+
 ### Commands ### {#module-browsingContext-commands}
 
 #### The browsingContext.activate Command ####  {#command-browsingContext-activate}
@@ -4563,7 +4574,7 @@ failed</dfn> steps given |context| and |navigation status|:
         browsingContext.UserPromptClosedParameters = {
           context: browsingContext.BrowsingContext,
           accepted: bool,
-          type: "alert" / "confirm" / "prompt" / "beforeunload",
+          type: browsingContext.UserPromptType,
           ? userText: text
         }
       </pre>
@@ -4614,7 +4625,7 @@ closed</dfn> steps given |window|, |type|, |accepted| and optional |user text|
           context: browsingContext.BrowsingContext,
           handler: "accept" / "dismiss" / "ignore",
           message: text,
-          type: "alert" / "confirm" / "prompt" / "beforeunload",
+          type: browsingContext.UserPromptType,
           ? defaultValue: text
         }
       </pre>