HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>computed vs resolved</title>
  <style>
    .container { width: 200px; height: 200px; }
    .item { width: 50%; height: 100px; background: tomato; }
  </style>
</head>
<body>
  <div class="container">
    <div class="item"></div>
  </div>
  <pre id="out"></pre>
  <script>
    var item = document.querySelector(".item");
    if (typeof item.computedStyleMap !== "function") {
      document.getElementById("out").textContent = "not supported";
    } else {
      var resolved = getComputedStyle(item);
      var computed = item.computedStyleMap();
      document.getElementById("out").textContent =
        "resolved width: " + resolved.width + "\n" +
        "computed width: " + String(computed.get("width"));
    }
  </script>
</body>
</html>

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Element.computedStyleMap — computed vs resolved width (Try It)