When referring to XML or SGML documents, the term Valid means that the document conforms to the DTD or Schema specified or referred to in the header.
and then sgmlnorm validates it against W3C's strict.dtd and tells me all my mistakes (having text that's not in <P> or <DIV> tags is one that I keep making). In Debian, this is in the sp package - "sgml parsing tools".
See also:
4 pages link to Valid:
lib/main.php:944: Notice: PageInfo: Cannot find action page