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chromium/third_party/blink/perf_tests/parser/query-selector-all-class-last.html 

<!DOCTYPE html>
<html>
<body>
<script src="../resources/runner.js"></script>
<script>
for (var i = 0; i < 1000; i++) {
    var div = document.createElement("div");
    div.id = "foo" + i;
    div.className = "bar" + i;
    document.body.appendChild(div);
}

var h1 = document.createElement("h1");
h1.id = "id";
h1.className = "class";
document.body.lastChild.appendChild(h1);

PerfTestRunner.measureRunsPerSecond({
    description: "This benchmark tests querySelectorAll() for an element that appears near the tail of the document, and is a child of an element with a given id",
    run: function() {
        for (var i = 0; i < 100; i++) {
            var res = document.querySelectorAll(".bar999 > h1");
        }
    }});
</script>
</body>
</html>