Browser version numbers continue to march on. IE9 is here, IE10 is coming, Firefox 5 and 6 are here with 7 and 8 in the wings, Opera's on 11, and Chrome is on, I dunno, somewhere between 14 and 50. Regardless, we'll all be on version 99 before The Singularity. There is a bug in the browser definition files that shipped with .NET 2.0 and .NET 4, namely that they contain definitions for a certain range of browser versions. But the versions for some browsers (like IE 10) aren't within those ranges any more. Therefore, ASP .Net sees them as unknown browsers and defaults to a down-level definition, which has certain inconveniences, like that it does not support features like JavaScript.