Checky Rocks!. Yet another reason to love Mozilla: the Checky plug-in. This thing gives you context menu access to 18 validator services, including 2 HTML validators, 2 CSS validators, 2 RSS validators, etc.
As if that wasn't good enough, you can set up Checky Agent with your favorite validation services; then, when you're on a page you want to validate, just hit F10 and all the validators are started on separate tabs. Brilliant!
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[The .NET Guy]
I didn't know about Checky... pretty cool.
3:38:23 PM