If you use tableless / CSS-based design methods, you can easily get back to text only.
You can use conditional comments to include only css files in a specific version of Internet Explorer.
You can use jquery version validation to run it only on the versions you want.
Something else that I just thought was that you can show the message to old browsers, as I saw across the Internet, urging the user to switch to compatibility and, most importantly, SECURITY. (I think Twitter is doing it now).
And one more thought: if you return to those browsers, you probably have to worry about screen size, since most sites are designed for at least 1024 x 768 these days, but immediately 800x600 was the initial one ...
rtpHarry
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