The first draft of Selectors Level 4 outlines a way to explicitly set the subject of a selector. This would allow the OP to style the list element with the selector $li > a.active
From Determining the Subject of a Selector:
For example, the following selector represents a list item LI unique child of an ordered list OL:
OL > LI:only-child
However the following one represents an ordered list OL having a unique child, that child being a LI:
$OL > LI:only-child
The structures represented by these two selectors are the same, but the subjects of the selectors are not.
Edit: Given how "drafty" a draft spec can be, it's best to keep tabs on this by checking the CSSWG's page on selectors level 4.
