I was thinking of something like this when I wrote my comment. MIME does function as a container model, but it's not plain text and it could only be a standard

I was thinking of something like this when I wrote my comment. MIME does function as a container model, but it's not plain text and it could only be a standard with supplementation. But there's a UNIX-y sense in which this is all human-readable text representations. I'm not really familiar with the internals of ODF, but OOXML, while it does capture quite a lot of UI state, is actually quite clean and represents the intuitive document structure rather well.