In theory, OOXML (Office Open XML) is an ISO/IEC standard (ISO/IEC 29500), despite heavy technical criticism being raised by many parties during the process and completely ignored by ISO/IEC. This shows that it is not a standard, let alone an open standard.

The following is a summary of these criticisms:

Complexity of specifications: the standard is extremely voluminous (~7,000 pages), making it virtually impossible for third parties to implement correctly. This contrasts sharply with competing standards such as ODF, which are much more concise.

Implementation inconsistencies: Microsoft Office applications do not implement the standardized version (ISO/IEC 29500 Strict), but use the “Transitional” variant, which includes compatibility features with legacy formats that contradict the stated goal of being a clean, modern, and above all open and standard format.

Proprietary dependencies: The specifications refer to several undocumented legacy behaviors of previous versions of Microsoft Office and require implementers to decode Windows-specific features to achieve compatibility.

Binary blob remnants: Despite being based on XML, OOXML incorporates binary data structures in many places, particularly for backward compatibility with legacy formats, and this compromises the transparency that XML should guarantee.

Platform-specific elements: The standard contains Windows-specific elements related to fonts, rendering, and other system behaviors that make any cross-platform implementation difficult or even impossible.

Controversy over the standardization process: The fast-track approval process adopted for OOXML by ISO/IEC was highly controversial, with allegations of procedural irregularities and vote manipulation raising legitimate doubts about the validity of the standard.