As of Windows Phone 7.5 (Mango), it's an improved version of the IE9 rendering engine, and is generally not bad. It doesn't work with a lot of mobile sites that

As of Windows Phone 7.5 (Mango), it's an improved version of the IE9 rendering engine, and is generally not bad. It doesn't work with a lot of mobile sites that expect WebKit (as with Opera Mobile or Firefox Mobile), but it's not a bad browser by any stretch.