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  手答え <em>tegotae</em>




                // create new attributeSheet
const attributeSheet = new CSSStyleSheet();

// replace all styles synchronously:
  div { 
    --lang: ja; 
  div em {
    --lang: en;

// Loop thru attributeSheet rules
[...attributeSheet.cssRules].forEach((rule) => {
  // Get elements that match the rule
  document.querySelectorAll(rule.selectorText).forEach((el) => {
    // Find all the properties/attributes
    const attributes =";");
    // Loop thru each attribute
    attributes.forEach((attr) => {
      if (attr.trim().length) {
        // Split attr-key and value
        const [key, value] = attr.split(":");
        // Set attribute and value
        el.setAttribute(key.trim().replace("--", ""), value.trim());