It's invalid according to the spec. They are permanently reserved code points for use in UTF-16. > The definition of UTF-8 prohibits encoding character numbers

It's invalid according to the spec. They are permanently reserved code points for use in UTF-16. > The definition of UTF-8 prohibits encoding character numbers between U+D800 and U+DFFF, which are reserved for use with the UTF-16 encoding form (as surrogate pairs) and do not directly represent characters. They could be replaced by the replacement character to produce a valid string.