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chromium/third_party/blink/web_tests/http/tests/webfont/webfont-performance-duration.html 

<!DOCTYPE html>
<script src="/js-test-resources/js-test.js"></script>

<div id="target">Hello webfont</div>

<script>
jsTestIsAsync = true;

var waitMillisBeforeFetching = 300;
var entry;
var fontFace = new FontFace('ahem', "url('../../../../misc/resources/acid3/font.ttf')");
document.fonts.add(fontFace);

new Promise(function(resolve) {
    setTimeout(resolve, waitMillisBeforeFetching);
}).then(function() {
    var target = document.getElementById('target');
    target.style['font-family'] = 'ahem';
    return fontFace.loaded;
}).then(function() {
    var name = document.location.origin + '/misc/resources/acid3/font.ttf';
    entry = window.performance.getEntriesByName(name)[0];
    shouldBeTrue('entry.startTime >= waitMillisBeforeFetching');
    finishJSTest();
});
</script>