March 26 is #DocumentFreedomDay, a day to acknowledge and celebrate the role of open standards in our lives.
Open standards are a common language that all computers can speak. When it comes to documents, spreadsheets and slide decks the premier standard is OpenDocument (.odt, .ods, .odp). If you use LibreOffice you may be familiar with these file extensions, if so you are already using OpenDocument.
Office Open XML (.docx, .xlsx, .pptx) is another standard created later, mainly used by Microsoft Office. It is actually a dual standard with two schemas, Strict and Transitional, the latter which allows proprietary (non-open) extensions. Only the latter transitional standard is in widespread use as it is the default save format of Microsoft Office. These proprietary extensions have been reverse engineered by others to create compatibility with Microsoft Office. That certainly doesn't sound very open to us.
You can read more at https://digitalfreedoms.org/en/dfd.