I spent some time investigating the IE9 render issues on branch-1 (and regression mentioned in HADOOP-7867). If I see things correctly, the cause of the regression is browsers failing to load the css file. Now that we changed the browser mode to HTML5, it fails to load the css file if the returned server content type does not match the expected value (in this case text/css).
The problem for this seems to be in HttpServer.java, where QuotingInputFilter#doFilter always sets the context type to text/html. This was fixed with
HADOOP-6496, so the missing piece is to backport this changelist. Will attach a patch for the backport.
Let me know if I should also rebase the changes to the latest trunk/branch-1.1, I'll be happy to do that.
Hope this helps