test_firstParty_postMessage.html

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<!DOCTYPE HTML>

<html>

<head>

  <meta charset="utf-8"/>

  <title>Test for Bug 1260931</title>

  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>

</head>

<script>

function onload() {

  let iframe1 = document.getElementById("iframe1");

  iframe1.contentWindow.postMessage("HI", "https://example.net");

window.onmessage = function(evt) {

  document.getElementById("message").textContent = evt.data;

  let iframe2 = document.createElement("iframe");

  iframe2.src = "dummy.html";

  document.body.appendChild(iframe2);

};

</script>

<body onload="onload()">

  <div>

    <iframe id="iframe1" src="test.html"></iframe>

    <span id="message"></span>

  </div>

</body>

</html>