<p>I'm getting warnings about CSS3.0 properties like text-overflow.  Is there a way to validate against 3.0?</p>
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<p>HTML5 and CSS3 support is coming to VS2010 in SP1.</p>
<p><a href="https://web.archive.org/web/20120621010521/http://blogs.msdn.com:80/b/webdevtools/archive/2011/01/27/html5-amp-css3-in-visual-studio-2010-sp1.aspx" rel="nofollow noreferrer">Link</a></p>
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<p>And now its here.</p>
<p><a href="http://visualstudiogallery.msdn.microsoft.com/7211bcac-091b-4a32-be2d-e797be0db210/view/Reviews/0?showReviewForm=True" rel="nofollow noreferrer">http://visualstudiogallery.msdn.microsoft.com/7211bcac-091b-4a32-be2d-e797be0db210/view/Reviews/0?showReviewForm=True</a></p>

Apparently, you can define your own Visual Studio Intellisense schema for CSS. I’m not sure if VS will then validate against that, or only use it for code completion.
Might be worth making a minimal one and seeing if it gets used for validation.
Custom CSS Intellisense Schema in Visual Studio 2005 and 2008
I don’t know of an easy way to validate against CSS 3 yet though. CSS 3 is a large, modular spec, and most of it is very much still in flux, despite some decent browser support for some properties.
http://www.w3.org/Style/CSS/current-work