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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>adoptedStyleSheets adopt</title>
</head>
<body>
  <p>Visit <a href="#">this link</a> — it should turn red.</p>
  <pre id="out"></pre>
  <script>
    if (!("adoptedStyleSheets" in document) || typeof CSSStyleSheet !== "function") {
      document.getElementById("out").textContent = "adoptedStyleSheets not supported";
    } else {
      var sheet = new CSSStyleSheet();
      sheet.replaceSync("a { color: red; }");
      document.adoptedStyleSheets.push(sheet);
      document.getElementById("out").textContent =
        "adoptedStyleSheets.length: " + document.adoptedStyleSheets.length;
    }
  </script>
</body>
</html>

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Document.adoptedStyleSheets — MDN adopt (Try It)