>I see the same results with your test.

At least that goes someway to confirming that it's nothing to do with the way my system is set up.

What I've found is that the list element numbers appear in the margin of the ordered list element, and are right justified. Any part of the numbers outside of the element border is not displayed, the solution is to increase the size of the margin of the < ol > element. Unfortunately, this requires prior knowledge of the maximum number in the list.