Here's an excerpt from the SGML declaration in ISO 8879 Annexes K and L:
Web SGML Adaptations(http://www.y12.doe.gov/sgml/sc34/document/0029.htm ),
[L.2] SGML Declaration for XML:

  DELIM
      GENERAL  SGMLREF
      HCRO     "&#x"
               -- Ampersand followed by "#x" (without quotes) --
      NESTC    "/"
      NET      ">"
      PIC      "?>"
      SHORTREF NONE

and the corresponding excerpt from HTML 4.01
(http://www.w3.org/TR/html4/sgml/sgmldecl.html ):

         DELIM    GENERAL  SGMLREF
                  HCRO "&#x" -- 38 is the number for ampersand --
                  SHORTREF SGMLREF

Do these mean that only a lower case "x" is allowed after "&#" in a
hexadecimal character reference open delimiter?  That is consistent with
this production in XML 1.0 (http://www.w3.org/TR/REC-xml#NT-CharRef ):

[66]    CharRef    ::=    '&#' [0-9]+ ';'
                          | '&#x' [0-9a-fA-F]+ ';' [WFC: Legal Character]

But it is inconsistent with the HTML 4.01 spec itself, which explicitly
allows "&#X" (http://www.w3.org/TR/html4/charset.html#h-5.3.1 ).

Thanks,
-- 
Kevin Rodgers