<html>
<head>
<script type="text/javascript">

function selectAndJustify()
{
    if (window.testRunner)
        testRunner.dumpAsText();

    var elem = document.getElementById("test");
    var selection = window.getSelection();
    selection.setBaseAndExtent(elem, 2, elem, 6);
    document.execCommand('JustifyCenter', false, null);
}

</script>
</head>

<body>
    <span id="test" contenteditable="true" >
        Line 1.
        <br>
        <b>Select all text in this line and use justify command.</b>
        <br>
        Line 3.
    </span>
</body>
<script>
selectAndJustify();
</script>
</html>