The Document Foundation (TDF), which backs LibreOffice, has renewed its criticism of proprietary file formats, arguing that Microsoft's Office Open XML (OOXML) formats keep users locked into Microsoft's own software despite carrying ISO certification. TDF contends that although OOXML is technically a standard, only Microsoft Office fully and correctly implements it, meaning the format functions as proprietary in practice even though it holds an official standardisation stamp. The group frames this as a matter of digital sovereignty, warning that reliance on such formats leaves institutions and individuals dependent on a single vendor.

TDF points to the split between OOXML Transitional and OOXML Strict, noting that Strict removed most problematic legacy elements but Microsoft defaults users to Transitional, burying Strict further down the options list or dropping it from some versions entirely. It argues genuine document independence requires deliberate choices around formats, such as adopting the OpenDocument Format used natively by LibreOffice, and even fonts, to avoid dependency on software that could later lose support, citing the impending retirement of Microsoft Publisher as a cautionary example. TDF acknowledges the practical difficulty organisations face in moving away from proprietary tools given the sheer volume of existing documents, but insists that continued reliance perpetuates dependency rather than genuine interoperability.