Which rule they violated?

I wonder if a case could be built on accessibility grounds actually...

Abuse of dominant market position in one market (search) to push into other markets (browsers).

Skype is a Microsoft product, though?

Sorry, I thought it was about hangouts or something.

Embarrassing, sorry, and thanks for pointing it out.

It's the same as games saying: "windows only". They could argue that it's a technical requirement.

In the "Windows only" case it is kind of reasonable: supporting another platform takes work.

When they are blocking other browsers explicitly despite the fact that the other platforms work that is a whole different level.

Well browsers with a different engine do work differently. By the way, I do disagree with this strategy. Just saying it is not necessarily something illegal.

Problem was in this case it seems the product works fine across different engines, they just decided it was ok to block a competing product to limit comsumer choices and generally make the web a worse place ;-)

Of course it doesn't have to be that black and white but they certainly makes it easy to dislike them at the moment.

In this case it's more like a game detecting that it's being run through Wine, and blocking it, even if it would have worked fine.