I wouldn't put .doc and .docx next to one another, as they're only tangentially related. I'd bet getting the <html><body><p>hello, world</p></body></html> of .docx would be some silliness, but would not be hard to grok. I couldn't readily find a browsable copy of ECMA 376 4th Ed online but https://github.com/PumasAI/WriteDocx.jl/blob/v1.2.0/docs/src... was in the ballpark of what I expected to find in some section of the actual spec
They are much more closely related than you might think - .docx is basically an XML serialization of the same object graph as .doc, but the latter is a COM Structured Storage file with binary objects inside. Anyway, most of the cognitive load would be in the semantics of various objects, not their representation.