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chromium/third_party/blink/web_tests/external/wpt/html/semantics/forms/the-button-element/button-activate.html 

<!doctype html>
<meta charset="utf-8">
<title></title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<iframe src="button-activate-frame.html" onload="runTest()"></iframe>
<script>
var t = async_test("button activation behaviour submits form");
function runTest() {
  var iframe = document.querySelector('iframe');
  iframe.onload = t.step_func(function() {
    t.done();
  });
  var doc = iframe.contentDocument;
  doc.querySelector('button').click();
}
</script>