Is there a site cataloging the different browser incompatibilities of major (Alexa top 100) sites? One client complained that one portion of the site wasn't rendering properly in IE6, even though that wasn't on our list of supported browsers. They said they doubt major sites like Amazon put out code that doesn't render properly in all browsers. I doubt that is true, but even if they do work around every browser quirk, they can spend more man-hours in a single day testing IE6 then it will take most small companies to develop an entire site.
I know sites like youtube are stopping support for IE6 and there are many calls to stop using IE6.
I know it's not possible to test every single quirk in every browser on every operating system. I would like a catalog of information of major sites that have incompatibilities to demonstrate these issues. How do you deal with customers who insist everything must work in outdated browsers?