Make sure your documents a rendered in standards mode and not in quirks mode if you want to use the latest features. Read more on http://msdn.microsoft.com/en-us/library/cc288325%28VS.85%29.aspx. So one way to ensure IE 9 is rendering your text/html document in standards mode is starting your documents with
<!DOCTYPE html>
<html>
...
</html>
If you use the F12 key then IE pops up its developer tools where you can also see and change the modes to experiment with them.