Although the Commission has declined to name the complainants -- and did not do so yesterday -- the most vocal about Windows RT has been Mozilla, the open-source developer whose Firefox is the world's second-most-popular Web browser.

Mozilla is particularly strong in Europe, where it has an estimated 29% of the market, behind only Google's Chrome. Its European share is considerably higher than the 18% it enjoys in the U.S., according to Irish metrics company StatCounter.

In July, Reuters reported that regulators were looking into allegations that full access to APIs (application programming interface) in Windows RT.

It wasn't a surprise: The Windows RT APIs had been a source of tension between Microsoft on one hand, and Mozilla and Google on the other, for months.

In May, Mozilla accused Microsoft of withholding APIs necessary to build a competitive browser for Windows RT, and said the behavior