Until now I had assumed the browser uses the Internet Explorer 9 rendering engine, as all alternate browsers in Marketplace.
When benchmarking Nokia Xpress however on html5test.com the browser scored 241 points + 2 bonus points, significantly higher than IE9 on Windows Phone, which scored 138 and 2 bonus points.
Having an alternate rendering engine on Windows Phone requires native code, something only OEMs usually have access to, and is an interesting development for Windows Phone 7. The development may give some users who have been clamouring for an IE alternative on Windows Phone a further incentive besides the data compression to use the Xpress app.
Thanks Shaik for the tip.