We do not claim that RadControls for ASP.NET AJAX pass CSS validation, only XHTML compliance. The problem is that older browsers (namely IE6/7) have bugs (like the hasLayout bug), do not support many of the modern browser features and also some modern browsers had chosen to introduce new CSS features as proprietary CSS properties (-webkit-something, -moz-something, etc.). To ensure matching cross-browser functionality in all our controls, we are forced to forfeit CSS validation. I hope this is not much of a problem for you.
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