Shortcomings of Microsoft's "Open" Office XML file format

I've read this file format described as placing angle brackets around the binary format. Apparently, the spec is underdefined, leaving many things out, so trying to match behavior and display becomes impossible.

They insist on the fact that, provided you make a valid use of the XML, pretty much changing the content of anything in an existing document can be achieved by sequentially 1) unzipping the content 2) making appropriate changes to one or more XML parts that are compatible with the provided XML schemas and open packaging relationships 3) zipping the content back .

Let's see if that's true.

Doesn't appear to be.