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chromium/third_party/blink/web_tests/fast/forms/select/selected-false.html 

<html>
<head>
<script src="../../../resources/js-test.js"></script>
<body> 
    <select size="1"> 
        <option id="opt1" value="1">1</option> 
        <option id="opt2" value="2">2</option>
        <option id="opt2" value="3">3</option> 
    </select>
    <div id="console"></div>
</body>
<script>
    var selectElement = document.getElementsByTagName('select')[0];
    var optionElements = selectElement.getElementsByTagName('option');

    shouldBe("selectElement.selectedIndex = 0; selectElement.selectedIndex", "0");
    shouldBe("optionElements[0].selected", "true");
    shouldBe("optionElements[1].selected", "false");
    shouldBe("optionElements[2].selected", "false");

    shouldBe("selectElement.selectedIndex = -1; selectElement.selectedIndex", "-1");
    shouldBe("optionElements[0].selected", "false");
    shouldBe("optionElements[1].selected", "false");
    shouldBe("optionElements[2].selected", "false");

    shouldBe("selectElement.selectedIndex = -2; selectElement.selectedIndex", "-1");

    optionElements[2].selected = true;
    optionElements[1].selected = true;
    shouldBe("selectElement.selectedIndex", "1");
    shouldBe("optionElements[0].selected", "false");
    shouldBe("optionElements[1].selected", "true");
    shouldBe("optionElements[2].selected", "false");
    
    optionElements[1].selected = true;
    optionElements[2].selected = true;
    shouldBe("selectElement.selectedIndex", "2");
    shouldBe("optionElements[0].selected", "false");
    shouldBe("optionElements[1].selected", "false");
    shouldBe("optionElements[2].selected", "true");

    optionElements[0].selected = false;
    optionElements[2].selected = false;
    shouldBe("selectElement.selectedIndex", "0");
    shouldBe("optionElements[0].selected", "true");
    shouldBe("optionElements[1].selected", "false");
    shouldBe("optionElements[2].selected", "false");

    optionElements[0].disabled = true;
    optionElements[0].selected = false;
    shouldBe("selectElement.selectedIndex", "1");
    shouldBe("optionElements[0].selected", "false");
    shouldBe("optionElements[1].selected", "true");
    shouldBe("optionElements[2].selected", "false");
</script>
</html>