表格背景色的问题

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<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<title>Untitled Page</title>
<script language=javascript>
var td
function onFcous()
{
if(td)
{
td.parentElement.style.backgroundColor="white"
}
var targetTD=window.event.srcElement
if(targetTD.tagName=="TD")
{
targetTD.parentElement.style.backgroundColor="PINK"
td=targetTD
}
}
document.onclick=onFcous
</script> 
</head>
<body>
    <table style="width: 547px" id="tab" border=1px>
        <tr>
            <td style="width: 179px; height: 21px">
                222</td>
            <td>
                444</td>
            <td style="width: 180px">
                333</td>
        </tr>
        <tr>
            <td style="width: 179px; height: 21px">
                43345</td>
            <td>
                333</td>
            <td style="width: 180px">
                444</td>
        </tr>
        <tr>
            <td style="width: 179px; height: 21px">
                345</td>
            <td>
                344</td>
            <td style="width: 180px">
                345453</td>
        </tr>
    </table></body>
</html>