CSS Dropdown: Fails

Why does this fail?

This should be read after looking at child.html.

The dropdown list is not part of the focussed entity because focus-within only focusses elements within the element it is applied to (as well as the element itself) and in this case the dropdwon list is a sibling of the li element which focus-within is applied to. So, when the links in the dropdown list are clicked the li element loses focus and so the dropdown list closes before the hyperlink can be activated.