W3C has tended towards lowercase for attribute names and values for a while.
For example section 4.11 of the xhtml 1.0 standard in 2002:
4.11. Attributes with pre-defined value sets
HTML 4 and XHTML both have some
attributes that have pre-defined and
limited sets of values (e.g. the type
attribute of the input element). In
SGML and XML, these are called
enumerated attributes. Under HTML 4,
the interpretation of these values was
case-insensitive, so a value of TEXT
was equivalent to a value of text.
Under XML, the interpretation of these
values is case-sensitive, and in XHTML
1 all of these values are defined in
lower-case.