Refused to apply style from... because its MIME type ('text/html') is not a supported style-sheet MIME type, and strict MIME checking is enabled
Stylesheets should have a MIME type of text/css. Your server is serving up a HTML document (probably a 404 NOT FOUND error page) when it should be serving a CSS document.
Solution:
In my case, the problem was an empty @GetMapping in one of my Controllers. I deleted it and the problem was gone.
I think the same answer will help someone who's having this similar problem and context ; e.g. JSP.
Background:
I had the same problem using Spring Boot and FreeMaker.
I've found that using a path that includes the domain, rather than just a relative path to the file, fixes the issue.
I.e:
src="https://<domain>.com/public/css/style.css"
rather than:
src="../../public/css/style.css"
I don't have a reason as to why this is the case. Perhaps something to do with the way static files are being served, or because it needs to be served over https. I'm using Firebase to host / serve.
Note that this works for both css and js files, and explicitly setting the MIME type didn't have an effect for me.