It's pretty interesting because in many cases the software evolved faster than anyone could keep up with, and there are large portions of backwards compatibility that I suspect were mostly accomplished through trial and error, especially when it comes to their layout algorithms.

When they opened up the Office file formats, they wrote an XML spec that corresponds to the fields in the various binary formats. And there are sections of it where the specification is simply, "this should be formatted the way MS Word 5.1 does it."