Test script with a CSS resource

css-resource-test.user.js

// ==UserScript==
// @name     CSS Resource Test
// @version  1
// @grant    GM.getResourceUrl
// @resource css test.css
// ==/UserScript==

(async function() {
var link = document.createElement('link');
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = await GM.getResourceUrl('css', 'text/css');
link.media = 'all';
(document.head || document.body).appendChild(link);
})();

test.css

body { outline: 1px dashed red; }

Ooof.

The stylesheet blob:http://example.org/db005dad-89f3-4bf3-869d-1a07cafc0ea2 was not loaded because its MIME type, “text/plain”, is not “text/css”.

This depends on the page! On some pages it works, on some pages they do something to make this strict.