I'm surprised they included proprietary format that's de facto standard in profession or supported by multiple tools (.xls, .xlsx) in preferred section [1]. I wonder if "well-known enough" is as good as "open" from preservation standpoint.

[1] https://www.loc.gov/preservation/resources/rfs/data.html

Especially when Office 365 shows that not even Microsoft is capable of making software which can display Office files anymore... if you have a Word file which was created or has ever been modified by the Word application, working with it through Office 365 in a browser is such a pain. I've literally had images which are impossible to delete or move in the web version, and they will absolutely render in the wrong place.

You can unzip the xlsx and read the xml inside. It’s not the worst format by far.