Well, probably Microsoft XPS, which was actually a fairly well designed format. But Microsoft didn't have the fight in them to really push it as a competitor to PDF. In part, I suspect b/c it's hard to justify investing a lot of money in your competing document standard as there is not much revenue you can derive from it. As of 2018, Microsoft no longer bundles XPS support in Windows 10.
XPS ultimately became an Open Standard as Open XML Paper Specification. But the fear and burn during IE era were far too great.
As part of a Java project (a while ago), I studied the RTF format, partly by reading the spec, and partly through reverse engineering - by creating multiple incrementally larger RTF docs, starting from zero content, then adding a word, then a font style, then a paragraph, a table, etc. And after each addition, opened the RTF in a hex editor and viewed the content, to help decipher the format rules. Then wrote a small RTF generation library in Java, that we used in the project to programmatically generate reports from DB data fetched via EJB. I also provided some ability to vary content and style independently. Good fun.
Unfortunately, it's not an open standard AFAICT.
[0] https://interoperability.blob.core.windows.net/files/Archive...
RTF is to Word like CSV is to Excel. In fact, we generated RTF because the output was to be input to Adobe InDesign.