I don't think it's the ie-specific stylesheet. I could be wrong though.
As SB65 said, you're using a header element, which IE8 and below don't inherently support. You're also using HTML5 elements without an HTML5 doctype declaration.
You also didn't close your header element anywhere. I don't even think you closed your wrapper div. I'd try to fix all these mistakes first. Use http://validator.w3.org/
to help you find some errors.