Since styles are usually included in your view or main page, VS cannot find them in your ViewUserControl. If you add something like the following to your ViewUserControl, you can work around the problem (and get intellisense) by not including CSS twice.
<% if (false) { %>
<link rel="stylesheet" type="text/css" ...
<% } %>
This will get intellisense since it can find the stylesheet, but using if (false) actually prevents it from being included at runtime.
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