what's the result for windows phone 7 IE compatibility test?
Answers should be specific with reference to what works and what doesn't.
Quirksmode says nothing.
EDIT: The test should consider css 2 & 3, DOM core, DOM HTML, DOM events, CSS Object model view, like in this site: http://www.quirksmode.org/compatibility.html
MORE: in the official blog there is nothing since may Link
MORE: the developers vm performs really bad in the acid tests http://picasaweb.google.com/lh/photo/RWVdCcbyRE1CRp5Eqn88YA?feat=directlink 5/100 on acid test 3
The browser in Windows Phone 7 is currently a stripped down version of the rendering engine from IE7 on the desktop and the javascript engine from IE8.
As you've seen in the ACID tests you linked to the browser scores very low in these.
If you are creating a web page/site that is specifically targetting the browser in WP7, my recommendation would be to keep the layout very simple. Imagine you're targetting an older feeature phone, rather than a smart phone.
It has been strongly hinted that a new browser will be released (OTA) to WP7 phones (hopefully early) in the new year. This newer browser will be based on IE9. Hopefully this will provide many missing features and have much greater handling for CSS.
For reference, PPK also has a mobile compatibility table at http://www.quirksmode.org/m/table.html. Again this still doesn't include WP7 and I wouldn;t have expected it to until it gains some marketshare.