The XML 1.1 recommendation as of 2006-12-05 lists some characters to avoid. This list includes
U+FDD0 through U+FDDF because they are not characters. (The Unicode standard even says they "are
not permitted for interchange".)
However, U+FDE0 through U+FDEF are the exact same, and the Unicode standard groups them together
with U+FDD0 through U+FDDF. Thus, the recommendation probably contains an error where the author
meant "[#xFDD0-#xFDEF]" rather than the stated "[#xFDD0-#xFDDF]".