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chromium/third_party/blink/web_tests/fast/dom/Window/device-pixel-ratio-on-zoom.html 

<!DOCTYPE html>
<html>
    <head>
        <script src="../../../resources/js-test.js"></script>
    </head>
    <body>
        <p>
            Tests that window.devicePixelRatio is updated on zoom.
        </p>
        <script>
            // testRunner.zoomPageOut/In zooms with a fixed factor of 1.2 instead
            // of the zoom levels of the actual browser.
            var ZOOM_MULTIPLIER = 1.2;

            function round(n)
            {
                return Math.round(n * 100) / 100;
            }

            var expectedRatio = window.devicePixelRatio;
            if (window.testRunner) {
                for (var i = 0; i < 5; i++) {
                    testRunner.zoomPageOut();
                    expectedRatio /= ZOOM_MULTIPLIER;
                    shouldBe('round(window.devicePixelRatio)', String(round(expectedRatio)));
                }
                for (var i = 0; i < 10; i++) {
                    testRunner.zoomPageIn();
                    expectedRatio *= ZOOM_MULTIPLIER;
                    shouldBe('round(window.devicePixelRatio)', String(round(expectedRatio)));
                }
            }
        </script>
    </body>
</html>