In number theory, we have a concept of an "equivalence classes", eg. what makes two numbers behave as equals when you do some operator like addition or multiplication. Two numbers are said to be equal when they give the same result under the operator. For instance, under binary addition, 1+1 = 2, but the integer 2 is in the same class as 0 with respect to binary addition, so 1+2=1.
XHTML documents are discrete chunks of data, that is, they are just really big numbers. (Those numbers happen to have a really complicated internal structure, but then again, so do integers, and reals, and complex numbers, vectors, matrices, tensors...) Browsers operate on those really big numbers to give us the browsing experience we love and hate so much. But IE6 gives you 0, zip, nada, nothing when you give it application/xhtml+xml arguments and so does IE9. So in a very fundamental sense with respect to Web standards, IE9 = IE6.