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Proposed API from bug 23475: document.focusedDialog: dialog group's selected dialog or null dialog.focusedDialog: dialog group's selected dialog or null document.focusedControl: control group's selected control or null dialog.focusedControl: control group's selected control or null element.focusOwner: focusable control's owner or null dialog.parentDialog: dialog group's manager or null
Definitely not adding this until <dialog> and the new focus mechanism are widely implemented, if ever.
Please file a new issue at https://github.com/whatwg/html/issues/new if there's still interest.