このプログラムのjavascriptが動いてないみたいで
透明化されないのですけどもどうしてでしょうか
おしえてくださいお願いしますm(__)m

<html>
    <head>
    <meta http-equiv="Content-Type"
          content="text/html; charset=shift_jis">
    <title>サンプル 02</title>
    <script language="javascript">
var opacity = 50;

function document_load() {
    fore.style.filter = "Alpha(Opacity=" + opacity + ")";
    label.innerHTML = "Opacity=" + opacity;
}
function dec_click() {
    if (opacity > 0) opacity -= 10;
    fore.style.filter = "Alpha(Opacity=" + opacity + ")";
    label.innerHTML = "Opacity=" + opacity;
}
function inc_click() {
    if (opacity < 100) opacity += 10;
    fore.style.filter = "Alpha(Opacity=" + opacity + ")";
    label.innerHTML = "Opacity=" + opacity;
}
    </script>
    </head>

    <body onLoad="document_load()">
    <table width="100%" height="100%">
    <tr><td align="center">
        <input type="button" onClick="dec_click()"
               value="より透明に">
        <input type="button" onClick="inc_click()"
               value="より不透明に">
    </th></tr>
    <tr><td align="center"><p id="label">Opacity=100</td></tr>
    <tr><td align="center" style="
        background-image:url(back00.jpg);
        background-repeat:no-repeat;
        background-position:center;
    ">
        <img src="fore00.jpg" id="fore">
    </th></tr></table>
    </body>
</html>