The browser on Windows Phone 7 is currently a stripped down version of the rendering engine from IE7 on the desktop and a javascript engine from IE8.
As you saw in the ACID tests, you were very low on browser performance.
If you are creating a webpage / website specifically designed for the browser in WP7, my recommendation would be to make the layout very simple. Imagine that you are targeting an older, but not a smartphone, but an older phone.
It was clearly outlined that a new browser will be released (OTA) on WP7 phones (hopefully early) in the new year. This new browser will be based on IE9. Hopefully this will provide many missing features and greatly simplify CSS handling.
, PPK http://www.quirksmode.org/m/table.html. , WP7, , .