I kind of remember a massive CDATA blob... but my memory must be playing tricks on me: Wikipedia shows some sample markup of the pre-2007 Microsoft Office (https://en.wikipedia.org/wiki/Microsoft_Office_XML_formats) and it doesn't look bad at all.
And it seems after 2007 they switched to an ECMA standard.
And it seems after 2007 they switched to an ECMA standard.