css_ready.js

const cssUrl = "/your.css";
const cssRequest = await fetch(cssUrl);
if (!cssRequest.ok) {
    throw new Error(`Failed to load '${cssUrl}' (status: ${cssRequest.status})`);
}
const cssText = await cssRequest.text();
const styleSheet = new CSSStyleSheet();
await styleSheet.replace(cssText);
document.adoptedStyleSheets = [styleSheet, ...document.adoptedStyleSheets];
// CSS is ready to be used now