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chromium/third_party/blink/web_tests/http/tests/linkHeader/resources/link-preconnect-frame.php 

<?php
header("Link: <   http://wut.com.test/>; rel=preconnect");
?>
<!DOCTYPE html>
I'm an iframe with a Link header
<script>
    if (window.testRunner)
        testRunner.notifyDone();
</script>