When it comes to the Acid3 test, the two areas where platform preview #4 falls short of 100% are said to be those that are in a state of flux. General support for SVG Fonts seems to be shifting to WOFF (Web Open Font Format), and the support for SMIL animation of SVG is "far from strong", according to Hachamovitch.
Microsoft has also provided several more demos to show developers some of the things that can be achieved in a modern browser, and how IE9 performs compared with the alternatives.
The company recommends web developers and designers begin testing their work in IE9 standards mode, send IE9 markup based on that used for other standard-based browsers (rather than on markup developed for earlier versions of Internet Explorer), and use feature detection rather than browser detection to cope with any differences in behaviour or features.
The preview build - and presumably the final version - requires Windows 7 or Vista. If you're keen, the platform preview and demos can be found here.