"Relaxed" is an easy to use HTML validator implementation
which doesn't use the official W3C DTD's. It rather validates
HTML documents using it's own schema definitions written in Relax NG
with embedded Schematron patterns. This is an
extremely expressive combination of languages which enables validation
of additional restrictions which can not be
expressed using DTD. This includes most restrictions specified in the
W3C HTML 4.01 and the W3C XHTML 1.0 recommendation
and some restrictions from WAI WCAG 1.0 Guidelines.