Before 2-3 days, while I was examining Bundling and Minification in ASP.NET 4.5. I observed problem with CSS3 minification while using default bundling which is available with Microsoft.Web.Optimization. Exploring more on it I found that there is a bug in CSS3 minification (while using nested parentheses) and it is already logged at
To use this transform type with Bundling and Minification download it from here and add downloaded class file into your ASP.NET 4.5 or MVC 4 application. You do not need to add reference to YUI Compressor.
Open Global.asax.cs file and in Application_Start event add following two lines.
DynamicFolderBundle Css3Bundle = new DynamicFolderBundle("css3", typeof(CSS3Minify), "*.css", false);
That’s it now you should get your CSS3 minified. However this is temporary workaround till this bug got fixed in next version of Microsoft.Web.Optimization.
Let me know if you found any problem with this.