If you would like to invest some time and disk space the definitive validator is the W3C online validator.
You can however also download a snapshot of the validator and run it yourself offline, you will need an instance of Apache Web server, Perl, some Perl libraries and the validator itself.
This guide explains how to install and run your own validator instance on Windows. For other platforms the installation guide gives details.
All the above is available Free, Gratis and Open Source - you are encouraged to feed back any bugs you spot and you are obliged by the GPL licence to give back any bug fixes and/or enhancements that you might make.
As an example I ran this page through the W3C validator and it came up with the following report: