In case you are using Less CSS to generate CSS markups, IIS won’t find your .less files by default. You will get a 404 Not Found error. The problem is the missing MIME type.
The solution is simple, just register the MIME type for .less in web.config:
<system.webServer>
<staticContent>
<mimeMap fileExtension=".less" mimeType="text/css"/>
</staticContent>
</system.webServer>
