The Platform Preview is an early look at the Internet Explorer 9 platform so some features are incomplete, some may change, and some may be added. Microsoft asks that you refrain from providing feedback on features where noted that they are either partially implemented or not available. Microsoft is aware of their condition and will provide updates in future releases. Similarly, for known issues, Microsoft is also aware of their existence and are actively working on them.
Using and setting the home page. The default home page of the Platform Preview is the Internet Explorer 9 Test Drive site, which you can use to see demos for and information about the new Internet Explorer 9 platform. You can press F10 at any time to go to your home page. To change your homepage, right click the Internet Explorer Platform Preview icon in the Start menu and choose Properties. In the Target field on the Shortcut tab add your home page URL to the end of the path.

new in Internet Explorer 9 Platform Preview 3
Features Available:
- IE9 User Agent String
- getElementsByClassName
- characterSet
- CSS3 Media Queries
- HTML5 Events
- DOMContentLoaded
- DOM Core
- DocumentType, ProcessingInstruction, createDocument
- DOM Traversal
- DOM L3 Events
- DOMAttrModified, Composition events

Features Partially Implemented :
- DataURI
- Support in script source

Developer Tools Features Available :
- User Agent String Picker
- A new tool that can be used to change the user agent string that IE sends with every request. The tool comes with some built in presets and new custom user agent strings can be added.
- Console Tab
- The console window is now a full tab and has been expanded to include the logging of diagnostic information from IE.
- Network inspection improvements
- Shortened URL
- Color coding for errors
- Scaling units (eg. 500 ms becomes 0.5 s