In your head section of the html document
<link rel="stylesheet" type="text/css" href="/path/to/ABCD.css">
Your css file should be the css file only and it should not contain any markup.
This is more likely the issue at the server side. The request style sheet page "Styles/Site.css" from your server of type "text/css", but the server might be responding to this request with the "test/html". I had this same issue when my server was running in the python and it was replying to the requested css files with header text/html (as that of my index.html file). So I re-arranged my server code and assigned a correct headers to its corresponding pages and the issue got resolved.
Review your .htaccess. Maybe, your .htaccess rule is interfering with your right output. So just try to browse your CSS resource directly in the address bar, it must be presented in a text format.