我看来，像这个问题是在您的IIS配置。它的配置可能提供与text / html的MIME类型.css文件。

Seems to me like the problem is in your IIS configuration. it might be configured to deliver .css files with text/html MIME type.

试着要去MIME类型配置Web服务器，看看你是否能发现任何东西在那里。

Try going to the MIME types configuration on the web server and see if you can spot anything there.

正确的MIME类型.css文件是文本/ CSS。

The correct MIME type for .css files is text/css.

您也可以对HTTP头的参数与一些HTTP嗅探器，如小提琴家看看。

You can also have a look on the HTTP header parameters with some HTTP sniffer such as fiddler.

更新：接受的答案应该是一个由@ breet，彭宁斯指出！
只是提供静态内容，该错误会自动消失：

Updating: The accepted answer should be the one pointed by @breet-pennings! Just providing static contents, the error vanished automatically: