copy css rules from a document to another one

copy-css-rules.js

// at src document
const rules = Array.from(document.styleSheets)
  .reduce((sum, sheet) => {
    // errors in CORS at some sheets (e.g. qiita)
    // like: "Uncaught DOMException: Failed to read the 'cssRules' property from 'CSSStyleSheet': Cannot access rules"
    try {
      return [...sum, ...Array.from(sheet.cssRules).map(rule => rule.cssText)];
    } catch(e) {
      // console.log('errored', e);
      return sum;
    }
  }, []);

// at dst document
const newSheet = document.querySelector('head').appendChild(document.createElement('style')).sheet;
rules.forEach(rule => newSheet.insertRule(rule, newSheet.length));