Selecting Every Second Level In CSS

Question

How would you select every second level in the stack of infinite levels using CSS only?

Sample code:

<div>
    1
    <div>
        2
        <div>
            3
            <div>
                4
                <div>
                    5
                    <div>
                        [...]
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

Visual example of the preferred result:

Note: no JavaScript assistance needed; there is no problem when the number of levels is known.

Answer 1

While there is no problem when the number of levels is known, when it is not known then it becomes impossible with CSS only, not even if you are able to anchor the topmost div to some other element with a selector.