Right! This sort of "ugly hack lore" more generally is why it's extremely difficult for third-party word processing applications to handle old Microsoft Word .doc files, and in fact is why Microsoft put so much effort into backward compatibility for modern versions of Windows: Microsoft Office actually contains the document-rendering engines of old versions of Microsoft Word, in order to handle documents which are in the old formats.

And I understand that a significant proportion of Azure is running VMs with headless copies of Word hooked up to Office 365 so that those same old documents can straightforwardly be opened online, too.

Apologies I can't find the reference (of course I read this on here).

I'm very curious if a) the VM stays open the whole time, b) your document gets exported to some kind of perfect serialization format that can then be re-saved when you're done, c) if Word is using something like operational transformations to store keystrokes and edits and then ticker-taping that into a save routine, or d) something else.