because its MIME type ('text/html') is not a supported stylesheet MIME type, and strict MIME checking is enabled.

This error message usually appears in the browser console when a CSS file is not loaded properly due to an incorrect MIME type. The MIME type of a file is a way of identifying the type of content that the file represents. In the case of a CSS file, the correct MIME type is "text/css".

If the server sends the CSS file with a different MIME type, such as "text/html", the browser will refuse to load it because it is not a supported stylesheet MIME type. This is often caused by a misconfiguration on the server.

To fix this issue, the server administrator needs to ensure that the correct MIME type is set for CSS files. This can usually be done by adding a line to the server configuration file or .htaccess file. For example, in Apache, you can add the following line to the .htaccess file:

AddType text/css .css

This tells the server to treat all files with a ".css" extension as CSS files with the correct MIME type. After making this change, the browser should be able to load the CSS file without any issues.