Mismatch between CSS and web-platform-tests semantics for reftests

web-platform-tests has a rather simple set of semantics for reftests;
each test is identified by a single (test, ref, type) tuple. Multiple
<link> elements in the same test file result in multiple tests with
different references.

By contrast the testthewebforward documentation suggests that CSS has a
rather more complex semantic; each test is of the form (test, ref_tree),
where ref_tree is a tree of (file, type) nodes and breadth in the tree
indicates multiple options and depth in the tree indicates multiple
requirements, so the overall test passes if all the comparisons down to
some leaf node pass. In concrete terms multiple <link> elements in the
same document represent multiple options, and each reference can <link>
to further references.

Do we know how necessary the more complex setup is? In particular I
think that the main distinguishing feature is the ability to specify
multiple references of which *any* may match (since the all must match
semantic is just a way of collapsing N tests down to a single test). Is
there a list of all tests that are using this feature?