Sorry not to mention this before. The markup produced by gradebook is missing the doctype and namespace declaration.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
This is not just academic, without it there is no guarantee that the user agents will operate in standards compliance mode - we cannot fix rendering bugs with assurance.