How would I convert a HTML5 / CSS3 document to PDF?

We're currently generating reports for our web application using html5 / css3, and they look good on screen, but obviously when the user hits print who knows what is going to come out of the printer. So, what I would like to know is what is the best way to convert these reports to PDF for download / printing while maintaining the same visual quality of the on screen reports.

Update 2010-10-26 16:01: We're using both .NET and Perl