This online version enables you use it without installing the client tool on your PC.
There is also a bug in PHP 5.2.13 and PHP 5.3.2 that will not allow URLs with dashes in them to validate.
If you are unsure of your PHP version you can run php -v from the command line if you have command line PHP installed, or create a PHP file with the following contents and navigate to it using your web browser: There is a lot of debate about whether using a regular expression is a good idea when doing URL validation.
More information about HTML Tidy is available from the original W3C page, and you can download a local copy of it from the Source Forge project page.
Please refer to the errata for this document, which may include some normative corrections.