Named entity references are valid in XML. They just need to be
declared
before they are used[1], unless they are one of the builtin named
entities < > ' " or & -- these are always valid
when
parsing with an XML parser.
Correct, it was an offhand comment and I skipped over all the
details. In general named entities other than the built-ins aren't
declared at the top of the file and many parsers don't bother to read
in external DTDs so any entities declared there aren't read and are
therefore considered invalid.
XHTML is XML, so if parsed by an XML parser, XML's builtin named
entities are available, and if the parser doesn't ignore external
entities, then the same set of (roughly) 250 named entities defined in
HTML are available as well[2].
Except that no browser that I know of actually reads in the XHTML DTD
when in standards compliant mode, so none of those entities are
actually viable to be used unless you include the declarations for
them at the top of every XHTML document (which is ludicrous).
The bottom line is that it's far, far better to use numeric entities
in XML and simply ignore all but the built-in named entities if you
want to have any confidence that the document will be parsed
correctly - hence my offhand comment.
Regards,
Adrian Sutton
http://www.symphonious.net
