HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>beforematch feature detect</title>
</head>
<body>
  <a href="#until-found-box">Try reveal</a>
  <div id="until-found-box" hidden="until-found">Hidden until found</div>
  <pre id="out"></pre>
  <script>
    function isBeforeMatchAvailable() {
      return "onbeforematch" in document.documentElement;
    }
    var out = document.getElementById("out");
    var box = document.getElementById("until-found-box");
    out.textContent = "beforematch available: " + isBeforeMatchAvailable();
    if (isBeforeMatchAvailable()) {
      box.addEventListener("beforematch", function () {
        box.textContent = "Revealed with beforematch support";
      });
    } else {
      out.textContent += "\nFall back: use a details/summary or expand button";
    }
  </script>
</body>
</html>

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Element beforematch — feature detect (Try It)