Actually, Arena does provide a limited capability of this kind.
If you click on the edit button, the source markup is annotated
with comment elements describing the errors and inserted at the
appropriate places.
-- Dave Raggett <dsr@w3.org> tel: +44 117 922 8046 fax: +44 117 922 8924
Hewlett Packard Laboratories, Filton Road, Bristol BS12 6QZ, United Kingdom