LibreOffice has been on the offensive lately, taking the time to call out Microsoft and its practices whenever it can. Now, it is at it again, accusing Microsoft of “intentionally” using “unnecessarily complex” file formats to achieve user lock-in with its Microsoft 365 (Office) documents.

For those who don’t know, XML is a markup language that programs like Microsoft 365 and LibreOffice use to structure and define documents.

As LibreOffice puts it:

An XML schema comprises the structure, data types and rules of an XML document and is described in an XML Schema Definition (XSD) file. This tells the PC what to expect and checks that the data follows the rules. In theory, XML and XSD together form the basis of the concept of interoperability.

The two office suites take very different paths here. LibreOffice uses the OpenDocument Format (ODF), an open standard meant to be controlled by no single company. This format gives us files like .odt for text and .ods for spreadsheets.

Microsoft, on the other hand, created its own Office Open XML (OOXML) to support every feature in its own software, giving us the familiar .docx and .xlsx. What’s interesting is that both formats are really just ZIP archives. The easiest way to verify this is to take a .docx file, rename it to .zip, and decompress it. This will show you the guts of a Microsoft 365 document.

As LibreOffice notes, XML is supposed to function as “a bridge,” but Microsoft is weaponizing its own schema by making it so “complex that it becomes a barrier rather than a bridge.” LibreOffice compares it to a railway system where the tracks are public, but one company’s control system is so convoluted that no one else can build a compatible train, making it almost impossible for others to compete. Passengers don’t realise they are being held hostage by these technical hurdles.