The section “J Suggestions for XML Names (Non-Normative)”  says:
# The first character of any name should have a Unicode property
of ID_Start, or else be '_' #x5F.
# Characters other than the first should have a Unicode property
ERROR: What does ID_Start and ID_Continue refer to? This is at least
not defined inside the XML 1.0 spec! My guess is that ID_Start should
have been 'NameStartChar'  while ID_Continue should have been 'Name'