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<head>
  <meta charset="utf-8">
  <title>registerProtocolHandler — template</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    function previewHandlerUrl(template, activatedHref) {
      if (!template.includes("%s")) {
        return { ok: false, reason: "missing-%s" };
      }
      return {
        ok: true,
        result: template.replace("%s", encodeURIComponent(activatedHref))
      };
    }
    var template = "https://mail.example.org/?to=%s";
    var activated = "mailto:webmaster@example.com";
    document.getElementById("out").textContent = JSON.stringify(
      previewHandlerUrl(template, activated)
    );
  </script>
</body>
</html>

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registerProtocolHandler — %s template (Try It)