Since IE is a Microsoft browser that only runs on Microsoft operating systems, my thinking is that this still falls under the second of my vectors, and is an extension of the operating system.
As part of the testing of each version of Windows, it makes sense that Microsoft would test all versions of IE under each version of Windows. Whether or not each browser version and each operating system version is supported, may be open to question or interpretation.
Alex / talexb / Toronto
Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.