Online validation service available at Page Valet.

mod_validator

mod_validator is an Apache module to validate markup. It now offers a choice of parsers:

OpenSP, the library that powers services such as Page Valet, Code Valet, the WDG Validator and the W3C Validator.
Xerces, the Apache Group's validating XML parser, used for WebÞing's older XML Validation service.

Compared to the W3C Validator it offers several advantages:

Features: better and more complete XML support offered by Xerces
Performance - no CGI overhead
Performance - no Perl overhead
Performance - no nsgmls overhead
Flexibility - generates results as XML
Accuracy - direct application of the parser, with minimal document sniffing applied only to support Appendix C XHTML.
Accuracy - no imprecise parsing of data from an external program.
Simplicity - the overall system size and complexity is much reduced.
Helpfulness - friendly error messages optimised for validation

As against that, by using XML it introduces an XSLT overhead (the cost of the additional flexibility), no effort has (yet) been made to make it easy to install, there are various things hardcoded in the source that shouldn't be, and the prerequisites are possibly more demanding:

Apache 2.0 (a port for Apache 1.3 should be very straightforward, but then browser support would be much more limited due to the absence of an XSLT Filter module).
OpenSP from current CVS (release 1.5 isn't quite enough).
Xerces-C 2.2
an SGML library and catalogue files for HTML and XML.
XSLT stylesheets, and to support a broad range of browsers an appropriate server-side processor such as mod_xml

Availability

Version 1.1

May 1^st. Although there are outstanding feature requests, it is three weeks since the last problem report was fixed. We are declaring this to be the first stable release.

mod_validator source code is available here under the GNU General Public License (GPL).