sort of? Nobody uses OOXML Strict, including Microsoft themselves. Office defaults to its own dialect of OOXML Transitional, i.e. standard plus vendor extension

sort of? Nobody uses OOXML Strict, including Microsoft themselves. Office defaults to its own dialect of OOXML Transitional, i.e. standard plus vendor extensions. This dialect also subtly differs between MSO versions. i.e. Microsoft is still playing silly buggers with file formats, same as it ever was.