The Safari Tidy plugin is a small plugin that lets you validate the webpages you browse for (x)html compliance. The actual validation is done by Tidy and the W3C validator. This plugin was initially modeled after a similar plugin for Firefox, which can be found here.
In the bottom right corner of each browser window you will see the amount of warnings and errors the current page has, along with a small icon.
If you enable the W3C validator, you'll get two sets of results. The W3C results are on the left, the Tidy results are on the right.
Clicking on the Tidy results will show the source of the current page. The default 'view-source' window of Safari is extended so that it shows all the errors and warnings in a list. The lines containing errors are highlighted so they can be located easily. Doubleclicking on a warning or error in the list will position the cursor at the correct location in the html source text.
Clicking on the W3C results will open a new browser window, containing the report generated by the W3C validator.
You can download the plugin here.