Sometimes you have classes in your HTML that are not in your CSS for one reason or another. Maybe they are only used in your JavaScript. If you are tired of Visual Studio giving you the warning "The class or CssClass value is not defined," there is a way to suppress it. Here's my solution:
First, add the following to the head element in your HTML document:
<head runat="server">
<title>The Page</title>
<asp:Literal Visible="false" runat="server">
<link href="~/Content/SuppressCssWarnings.css" rel="stylesheet" type="text/css" />
</asp:Literal>
<!-- You probably have more here... -->
</head>
Notice, a few things here. First, that there is a runat="server" in the head. This is important for the ~ to work correctly. I also think you need it in order to have any <asp: ...> control in the head. The next thing to notice is the css file that has been added. This is where you add all the classes that Visual Studio is warning you about. Just cut and paste, it's not that bad. Finally, see how the link to your stylesheet is inside the asp Literal control? That keeps it from showing up in your rendered page.
Inside the SuppressCssWarnings.css file is just a list of your classes like so:
/* The whole point of this css is to get VS off my back. I'm sick of its "The class or CssClass value is not defined." */
.yui-skin-sam {}
.yuimenubar {}
.yuimenubarnav {}
.bd {}
.yuimenubaritem {}
.first-of-type {}
.yuimenubaritemlabel {}
.yuimenu {}
.yuimenuitem {}
And that's it! Good luck! As you can see I've been working with YUI recently. Check it out, it's a good JavaScript library, and it should work especially well the the ASP.NET MVC Framework.