See attached.
Created attachment 5715 [details] here's a first try - this fails two layout tests
Created attachment 5863 [details] ok, here's a version that works
Created attachment 5864 [details] ok, here's a version that works
Comment on attachment 5864 [details] ok, here's a version that works Oops, accidentally attached twice
Comment on attachment 5863 [details] ok, here's a version that works Gave Maciej a few comments on IRC, nothing major. r=me