Export documents in Office Open XML strict format

Dear all,

I’m working in a patent law firm and we are thinking about switching from MS Office.

Lately patent offices are switching to XML files for submissions and they now started to switch to Office Open XML strict (not transitional!). As for the specific requirements, see for example here:
https://www.epo.org/en/legal/official-journal/2025/07/a50

I know that this standard is not really “open”, but that is what we have to live with. Our professional body is trying to convince the European Patent Office to also accept ODT (see epi Information | Report of the Online Communications Committee), so please no response that we shouldn’t use that standard without answering my question, we are trying to move to ODF, but we also have to play with the cards we’re dealt with.

As the go live for OOXML is in April this year, my question would be whether LibreOffice is capable of exporting documents in OOXML (strict)? I’ve only seen Word 2010-365 or Word 2007, but don’t know if one of these or both are OOXML transitional or one is the strict version.

Thank you for your help!