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chromium/third_party/blink/web_tests/external/wpt/html/semantics/forms/form-submission-target/rel-button-target.html 

<!DOCTYPE html>
<title>&lt;form rel> with &lt;button formtarget></title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src=resources/reltester.js></script>
<div id=log></div>
<form action=resources/endpoint.html><input type=hidden name=channelname><button type=submit formtarget=_blank></form>
<script>
const submitter = document.querySelector("button"),
      channelInput = document.querySelector("input");
relTester(submitter, channelInput, "<button formtarget>");
</script>