Well in this fantasy TDD scenario (as in we don't know what happened in MS so I'm just making stuff up), presumably the product requirement was "make IE9 look v

Well in this fantasy TDD scenario (as in we don't know what happened in MS so I'm just making stuff up), presumably the product requirement was "make IE9 look very fast on the benchmarks without getting caught cheating". So sure, they solved the first failing unit test (make IE9 look quick), but don't seem to have written enough unit tests to make sure the second part of the requirement works. So they would fail the acceptance tests and have to keep working on it. (Wouldn't count myself as a full on TDD proponent but do use it when the time is right.)