StyleSheetList

for (const styleSheet of document.styleSheets) {
  console.log(styleSheet); // A CSSStyleSheet object
}

const allCSS = [...document.styleSheets]
  .map((styleSheet) => {
    try {
      return [...styleSheet.cssRules].map((rule) => rule.cssText).join("");
    } catch (e) {
      console.log(
        "Access to stylesheet %s is denied. Ignoring…",
        styleSheet.href,
      );
      return undefined;
    }
  })
  .filter(Boolean)
  .join("\n");