1. Don't write OOXML at all
2. Write OOXML *perfectly*
Currently, LibreOffice implements a third solution:
3. Write OOXML 'pretty well'
This last option gives us the worst of both worlds: it further establishes OOXML as the standard format, and it further establishes MS Office as the only office suite that supports the standard format perfectly.
(With "OOXML", I mean the format that MS implemented, not the one that was specified as an ISO standard.)