chromium/src/third_party/blink/web_tests/http/tests/misc/percent-sign-in-form-field-name.html

<form method="post" action="resources/percent-sign-in-form-data-field-name.cgi" enctype="multipart/form-data">
<input type="text" name="abc%40def" value="test">
</form>
<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}
document.forms[0].submit();
</script>