I\'ve found out several topics related to this issue yet I couldn\'t find an answer which worked for my problem, which is following: I moved my Asp.Net 4.0 site to a new server
Please Make sure Your IIS 'Static Content' is enabled. Do to This follow these steps:
Hopefully it will help you.
The process of enabling Static Content on Windows 7 is different from that on Windows Server 2008.
To install the Static Content feature on Windows Server 2008 and Windows Server 2008 R2, follow these steps:
a. Open Server Manager, and then expand Roles. b. Right-click Web Server (IIS), and then click Add Role Services. c. Under Web Server, click to select the "Static Content" check box. d. Click Next to complete the installation.
Here is the complete step by step guide for both situation for reference. http://support.microsoft.com/kb/2196177
The articles/topics that you have found is talking about the same thing as Uali's answer, which is correct, but it doesn't always solve everyone's problem. The problem is also not about your css per se.
Before proceeding, make sure that you have enabled Static Content as the other answer instructed.
Then do these steps in the IIS Manager window with your virtual directory selected.
Update: I also forgot to say that you can do the same thing for .jpg, .png, .gif or any other file types if necessary.