This is an outside shot without your code, but the solution when I had the exact same problem. My DOCTYPE statement had gotten fouled up by an errant keystroke, as described in this question. You might want to inspect it visually and/or validate.
This is an outside shot without your code, but the solution when I had the exact same problem. My DOCTYPE statement had gotten fouled up by an errant keystroke, as described in this question. You might want to inspect it visually and/or validate.