Jim Thatcher has posted details of the file format options in the next version of Office and its support of IS29500 strict.
From his post, the following table shows the evolution of file format support in the last few versions of Office:
The New Office
Binary format (.doc, .xls, .ppt)
Open, Edit, Save
Transitional Open XML
Strict Open XML
Open, “Edit”, Save
It is definitely worth your time to read his post in detail.