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chromium/third_party/blink/web_tests/external/wpt/dom/nodes/Document-contentType/contentType/contenttype_datauri_02.html 

<!DOCTYPE html>
<title>Data URI document.contentType === 'text/html' when data URI MIME type is set</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id=log></div>
<script>
async_test(function() {
  var iframe = document.createElement('iframe');
  self.onmessage = this.step_func_done(e => {
    assert_equals(e.data, "text/html");
  });
  iframe.src = "data:text/html;charset=utf-8,<!DOCTYPE html><script>parent.postMessage(document.contentType,'*')<\/script>";
  document.body.appendChild(iframe);
});
</script>