:D Did not know about Opera Mini.
So they put engine outside of the box :)
Anyway. To make modern (fast!) rendering engine you need a lot of specialized (and potentially harmful, or energy inefficient) APIs.
Lack of them put 3rd party web browser vendors at disadvantage compared to OS manufacturer (who can use those API..).
It also is valid for other applications but web browsers tend to use state of the art tech. And there is always demand for more of it.
MS limitation harm competition there.