http://bdash.net.nz/files/arguments-combinations.html is a test case using InsertOrderedList rather than InsertHorizontalRule. You'll notice that a test is failing as InsertOrderedList does not insert id="" when the empty string is passed as the value argument. According to the HTML 5 specification an empty id attribute is not valid. I believe that InsertHorizontalRule should match InsertOrderedList/InsertUnorderedList on this point.
Created attachment 17419 [details]
Comment on attachment 17419 [details]
I think a test of InsertOrderedList and InsertUnorderedList with an empty string 3rd parameter would be nice if it doesn't already exist.
Landed in r27931.