Clarify resolution and default of URL-related manifest members (mdn#3…

…4638)

files/en-us/web/manifest/id/index.md

@@ -18,6 +18,10 @@ browser-compat: html.manifest.id
 
 The `id` member is a string that represents the _identity of the web application_ — the unique identifier for the web application. If the web application ID does not match an existing ID, the application will be treated as a unique identity even if it is from the same URL.
 
+A valid `id` needs to be same-origin with `start_url`. If the `id` is a relative URL, it is resolved using the origin of [`start_url`](/en-US/docs/Web/Manifest/start_url). The fragment is always ignored.
+
+If the `id` is unspecified or invalid in any way (such as not a string, not a valid URL, not same-origin with `start_url`), the `start_url` is used.
+
 > **Note:** The `id` member can be used to uniquely identify applications. The `id` member is processed like a `start_url` but can be out of [scope](/en-US/docs/Web/Manifest/scope) because it does not point to a resource that can be navigated to.
 
 ## Examples

files/en-us/web/manifest/scope/index.md

@@ -18,7 +18,9 @@ browser-compat: html.manifest.scope
 
 The `scope` member is a string that defines the navigation scope of this web application's application context. It restricts what web pages can be viewed while the manifest is applied. If the user navigates outside the scope, it reverts to a normal web page inside a browser tab or window.
 
-If the `scope` is a relative URL, the base URL will be the URL of the manifest.
+A valid `scope` needs to allow [`start_url`](/en-US/docs/Web/Manifest/start_url) to be in scope. If the `scope` is a relative URL, the base URL will be the URL of the manifest. The query and fragment of the URL are always ignored.
+
+If `scope` is unspecified or invalid in any way (such as not a string, not a valid URL, or causes `start_url` to be out of scope), the `start_url` is used, with its last path segment removed.
 
 ## Examples
 

files/en-us/web/manifest/start_url/index.md

@@ -18,6 +18,8 @@ browser-compat: html.manifest.start_url
 
 The `start_url` member is a string that represents the _start URL of the web application_ — the preferred URL that should be loaded when the user launches the web application (e.g., when the user taps on the web application's icon from a device's application menu or homescreen).
 
+A valid `start_url` needs to be same-origin with the document that references the manifest. If `start_url` is unspecified or invalid in any way (such as not a string, not a valid URL, or not a same-origin with the document), the document URL is used.
+
 > **Note:** The `start_url` member is purely advisory, and a user agent may ignore it or allow the user to alter it at install time or afterwards.
 
 ## Examples