A web page may implement their own key command handling and want to prevent the default browser behavior by calling Event.preventDefault() in their onkeydown handler. For example, a page may implement their own Find-in-page feature that is toggled by listening for Command + F, the same key command to trigger the default browser Find-in-page feature. Calling Event.preventDefault() in their onkeydown handler when they receive Command + F should prevent Safari from showing the Find-in-page UI. But it does not on iOS. It does on Mac.