What is the issue with the HTML Standard?

Looking at the same-origin propagation WPT, it has a frame tree like this (simplified):

• top
  • child one
  • child two

It clicks child two, then expects child one to be isActive and hasBeenActive too.

The spec user activation processing model states:

2. Let windows be « document's relevant global object ».
3. Extend windows with the active window of each of document's ancestor navigables.
4. Extend windows with the active window of each of document's descendant navigables, filtered to include only those navigables whose active document's origin is same origin with document's origin.
5. For each window in windows, set window's last activation timestamp to the current high resolution time.

It does not state what to do with the elements siblings (seemingly excluding them), yet the WPT expects the sibling of child two (child one) to be activated. Chromium and WebKit appear to follow the WPT behavior and not spec as above, this should probably be corrected in spec by adding same-origin siblings to windows too.