[focus-without-user-activation] Add pseudo algorithm and examples

policies/focus-without-user-activation.md

@@ -34,6 +34,21 @@ In a nutshell:
   * Before starting [steps](https://html.spec.whatwg.org/multipage/interaction.html#dom-window-focus) for `element.focus(options)` the same verification for the policy and user activation should be performed.
   * Around step 2 of the [spec](https://html.spec.whatwg.org/multipage/interaction.html#dom-window-focus) for `window.focus()`, the same enforcement should be made (using the browsing context of the `window` itself to obtain the permissions policy state).
 
+Pseudo-algorithm for how the policy would integrate with focus:
+
+```
+algorithm is_allowed_to_set_focus(focus_setter_frame, currently_focused_frame):
+  if focus_setter_frame has the policy allowed:
+    return true
+
+  if currently_focused_frame is an inclusive descendant frame of focus_setter_frame:
+    return true
+
+  return false
+```
+
+Note: An [inclusive descendant](https://html.spec.whatwg.org/#inclusive-descendant-navigables) frame is a frame that is either the same frame or a descendant frame in the frame tree hierarchy.
+
 Using the Feature
 -------------
 This feature can be introduced with the HTTP headers. For instance,
@@ -50,11 +65,6 @@ To disable the feature for a specific `<iframe>`, the `allow` attribute can be u
 which would block use of focus (without activation) for the document inside the `<iframe>`
 unless it is a same-origin document.
 
-The Extra Mile
------------
-Automatic focus, in general, poses security concerns. It might be a good idea to disable this policy
-in all sandbox-ed frames (treat the policy as a sandbox flag).
-
 Alternative Solutions Considered
 -----------
 This section lists other possible solutions that were considered during the development of the proposal outlined in this explainer.
@@ -85,3 +95,21 @@ This section lists other possible solutions that were considered during the deve
 3. **Sandbox flag approach**: The possibility of implementing this control as a [sandbox](https://developer.mozilla.org/en-US/docs/Web/API/HTMLIFrameElement/sandbox) flag was analyzed instead of a permissions policy.
 
    Adding this functionality to the sandbox would be potentially breaking, as it would immediately affect every sandboxed frame and require all sites to update their code if they needed to restore the functionality. In contrast, implementing this as a permissions policy is non-breaking: with a default allowlist of `'self'`, it provides an opt-in control mechanism that is enabled by default everywhere but can be selectively disabled when needed.
+
+Appendix
+-----------
+
+These are some example cases of how focus setting would now work with this new policy in the pseudo-algorithm described above:
+
+| Case | Policy Enabled on `focus_setter_frame` | `focus_setter_frame` | `currently_focused_frame` | Allowed to Set Focus? | Reason |
+|------|----------------------------------------|------------------------|----------------------------|------------------------|--------|
+| 1    | No                                  | Parent                 | Child                      | Yes                 | Parent-child relationship is allowed by default. |
+| 2    | No                                  | Child                  | Parent                     | No                  | No policy and not a permitted direction. |
+| 3    | No                                  | Grandparent            | Grandchild                 | Yes                 | Ancestor allowed to set focus when a descendant has it. |
+| 4    | No                                  | Grandchild             | Grandparent                | No                  | No policy and not a direct relationship. |
+| 5    | No                                  | Same frame             | Same frame                 | Yes                 | A frame is always allowed to set focus on (maybe another element of) itself if it already has focus. |
+| 6    | Yes                                 | Parent                 | Child                      | Yes                 | Policy allows it explicitly. |
+| 7    | Yes                                 | Child                  | Parent                     | Yes                 | Policy allows it explicitly. |
+| 8    | Yes                                 | Grandparent            | Grandchild                 | Yes                 | Policy allows it explicitly. |
+| 9    | Yes                                 | Grandchild             | Grandparent                | Yes                 | Policy allows it explicitly. |
+| 10   | Yes                                 | Same frame             | Same frame                 | Yes                 | Policy allows it explicitly. |