Fine to leave focus in for now (ref #80), but I have the following concerns about it:

• User privacy: This is an unambiguous indication of assistive technology where mere navigation of the content "outs" the user.
• Compatibility with non-linear navigation methods: Would it still be called "focus" if it's triggered by something that would fire many more events that linear keyboard focus? E.g. Pointer-based dwell AT may need to fire this on hover, more akin to mouseover/mouseout.
• Robustness: To make this complete, we'd need a pair of actions: focus/blur, or focusIn/focusOut.
• Ambiguity: As you're well aware (see :focus-ring) ambiguity between different types of focus can result in misuse or cumbersome workarounds by web developers.

Thoughts:

• Should actions allow an optional payload (e.g. "type of focus" or any other relevant data)
• Should a user privacy model be included at this point in the Incubation process? E.g. prompt the user (non-blocking) when the page first registers for actions. Consider aspects of the privacy model I sketched up for IndieUI User Context a few years back.