The Document Foundation, the nonprofit behind LibreOffice, has issued a sharp critique of Microsoft’s Office Open XML file formats, arguing that an ISO certification sticker does not make a format truly open when only one company’s software renders it correctly.
OOXML, the .docx, .xlsx, and .pptx formats used by hundreds of millions of people daily, carries ISO 29500 certification. But The Document Foundation contends that the standardization process was “one of the most contested in the history of ISO” and that the result is a specification describing not an interoperable format but “the existing behavior of Microsoft Office, including legacy behaviors, undocumented features, and implementation details specific to Microsoft’s source code.”
Two standards, one used
The most concrete problem is that OOXML exists in two variants. OOXML Transitional contains legacy elements, undocumented behaviors, and format quirks preserved from earlier versions of Office. OOXML Strict is a cleaner variant that removed most problematic items and could be implemented by other software. Microsoft defaults to Transitional and hides Strict.
“The practical effect is that the format used daily by hundreds of millions of people is the one that only Microsoft’s own software implements correctly,” The Document Foundation said, “while the cleaner variant, which other software could actually support, is not used, and has now even disappeared from some versions.”
“A standard that only one implementation fully supports is, from a functional point of view, a proprietary format with a standardization certificate,” the organization concluded.
Lock-in at every layer
The critique extends beyond file formats. The Document Foundation argues that dependency on proprietary fonts and software creates a compounding lock-in effect. “When an institution sends a letter formatted with a proprietary font, embedded in a proprietary format, produced by proprietary software, it is not communicating information but perpetuating a dependency.”
Italo Vignoli, a Document Foundation board member, provided migration strategies for large organizations with millions of documents: batch-convert with LibreOffice and manually check results, convert only actively used documents and archive the rest as PDF, or a combination of both, “usually the chosen one for large organizations.” Vignoli drew on experience from migrating half a million seats from Microsoft Office to LibreOffice.
Digital sovereignty stakes
The issue has direct implications for government digital sovereignty. The Register reported that The Document Foundation has previously urged the EU to ditch Excel lock-in for cybersecurity law consultations. With Microsoft Publisher retiring in 2026, the risk of document orphanage is not theoretical, institutions relying on proprietary formats face the prospect that old documents become unopenable.
The Document Foundation recommends organizations adopt the OpenDocument Format, which LibreOffice uses natively, as a genuinely interoperable alternative.
Microsoft had not responded to The Register’s request for comment at time of publication.
Sources: The Document Foundation says Microsoft’s file formats keep users on a short leash (The Register, July 2026)