Number of documents written in standardised OOXML? Practically zero: https://social.technet.microsoft.com/Forums/en-US/e969fc0a-9fcd-4efe-bf6d-79ea8c34360f/what-is-the-default-file-format-for-saving-in-ms-office-2013-is-it-still-the-transitional-ooxml-or
MS Office 2010 and below don't even support the standardised OOXML! Even the standardisation process was fast-tracked and very controversial. Besides the ODF there are very few documents in existence that follow open standards, and so there is no way to achieve "widest compatibility" without resorting to proprietary formats. It's a very unfortunate situation. Microsoft's continued hostility to open standards has gone a long way... Andrew.
