When we turn on "usexml " in the "print_header_simple" call function (in the view.php of an activity), there are 2 declarations of DOCTYPE which cause a bug.

The first declaration is in the "print_header" function (when use the xhtml+xml option), the second in the top of "header.html".
I think the first declaration may be deleted.
 "print_header" is in the weblib.php file

Speaking at http://moodle.org/mod/forum/discuss.php?d=101255