Unlike it major competitors, Microsoft's Windows Phone 7 will not support HTML 5. Instead, the browser will be based upon Internet Explorer 7.0, said Larry Lieberman, senior product manager for Windows mobile. Microsoft is making its initial bet on Silverlight and XNA applications. Without HTML 5 support, alternatives for developing Web applications are more limited.
Incidentally, IE7 failed the Acid2 test, (which is now 5 years old) but Lieberman said that it would be comparable with other mobile browser experiences. iPhone Safari, Android Chrome fair much better. The Internet Explorer 8 browser does pass Acid2. When asked why Microsoft opted to use a dated browser, Lieberman explained that Microsoft has focused its resources on breaking with the past to provide a better user experience through Silverlight.