The border collapsing algorithm ignores column group borders if the <colgroup> has <col> children. This is why tables/mozilla_expected_failures/core/table_rules.html and tables/mozilla_expected_failures/marvin/x_table_rules_groups.xml fail. Patch forthcoming.
Created attachment 15239 [details] Check if the col element is inside a colgroup
Comment on attachment 15239 [details] Check if the col element is inside a colgroup r=me
Landed in r23810.