When they have a monopoly, places like the EU will frown on purposefully breaking compatibility.

Its called antitrust.

> When they have a monopoly, places like the EU will frown on purposefully breaking compatibility.

What exactly have they done about it?

Without knowing too much of the EU history, I have always understood that anti-trust pressure from the EU effectively forced Microsoft to publish the OOXML spec in the first place.

> Purposefully

According to who? With what proof? And how/why do they get to be the arbiters of that?

> According to who? With what proof?

They normally get asked to investigate by other interested parties, and then ask other independent experts in the field.

> And how/why do they get to be the arbiters of that?

By being the government?

Microsoft doesn't have to sell their software in Europe if they don't like the rules.

no, only if you have a quasi monopoly on Office Application in pretty much every single (western) government through all departments and sectors.