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chromium/third_party/blink/web_tests/fast/dom/HTMLLinkElement/link-preconnect-valid.html 

<!DOCTYPE html>
<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
        testRunner.dumpChildFrames();
    }
    if (window.internals)
        internals.settings.setLogDnsPrefetchAndPreconnect(true);
</script>
<link rel="preconnect" href="http://prefetch.wut.com.test">
This test verifies that a DNS prefetch request is sent and logged.