Office Open XML (docx), to be fair, is a non-proprietary standard. Microsoft has been forced into that by the EU; and they keep messing with it, but it is actually technically "open".
[1] https://osswatch.jiscinvolve.org/wp/2007/01/05/criticism-of-...
[2] http://www.robweir.com/blog/2007/01/how-to-hire-guillaume-po...
[3] https://en.m.wikipedia.org/wiki/Standardization_of_Office_Op...
It would be a monumental effort to create another implementation and completely impossible without referring to MS Office as a reference implementation.