Hi ,

You can use this ..

<apex:page standardController="Account" showHeader="True" sidebar="false" tabStyle="Account" >
<style type="text/css">

#modalContainer {
    background-color:transparent;
    position:absolute;
    width:100%;
    height:100%;
    top:0px;
    left:0px;
    z-index:10000;
    background-image:url(tp.png); /* required by MSIE to prevent actions on lower z-index elements */
}

#alertBox {
    position:relative;
    width:300px;
    min-height:100px;
    margin-top:50px;
    border:2px solid #000;
    background-color:#F2F5F6;
    background-image:url(alert.png);
    background-repeat:no-repeat;
    background-position:20px 30px;
}

#modalContainer > #alertBox {
    position:fixed;
}

#alertBox h1 {
    margin:0;
    font:bold 0.9em verdana,arial;
    background-color:#78919B;
    color:#FFF;
    border-bottom:1px solid #000;
    padding:2px 0 2px 5px;
}

#alertBox p {
    font:0.7em verdana,arial;
    height:50px;
    padding-left:5px;
    margin-left:55px;
}

#alertBox #closeBtn {
    display:block;
    position:relative;
    margin:5px auto;
    padding:3px;
    border:2px solid #000;
    width:70px;
    font:0.7em verdana,arial;
    text-transform:uppercase;
    text-align:center;
    color:#FFF;
    background-color:#78919B;
    text-decoration:none;
}

/* unrelated styles */

#mContainer {
    position:relative;
    width:600px;
    margin:auto;
    padding:5px;
    border-top:2px solid #000;
    border-bottom:2px solid #000;
    font:0.7em verdana,arial;
}

h1,h2 {
    margin:0;
    padding:4px;
    font:bold 1.5em verdana;
    border-bottom:1px solid #000;
}

code {
    font-size:1.2em;
    color:#069;
}

#credits {
    position:relative;
    margin:25px auto 0px auto;
    width:350px; 
    font:0.7em verdana;
    border-top:1px solid #000;
    border-bottom:1px solid #000;
    height:90px;
    padding-top:4px;
}

#credits img {
    float:left;
    margin:5px 10px 5px 0px;
    border:1px solid #000000;
    width:80px;
    height:79px;
}

.important {
    background-color:#F5FCC8;
    padding:2px;
}

code span {
    color:green;
}
</style>
<script type="text/javascript">

/************************************

    Custom Alert Demonstration
    version 1.0
    last revision: 02.02.2005
    steve@slayeroffice.com

    Should you improve upon this source please
    let me know so that I can update the version
    hosted at slayeroffice.

    Please leave this notice in tact!

************************************/

var ALERT_TITLE = "Oops!";
var ALERT_BUTTON_TEXT = "Ok";

if(document.getElementById) {
    window.alert = function(txt) {
        createCustomAlert(txt);
    }
}

function createCustomAlert(txt) {
    d = document;

    if(d.getElementById("modalContainer")) return;

    mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
    mObj.id = "modalContainer";
    mObj.style.height = d.documentElement.scrollHeight + "px";
    
    alertObj = mObj.appendChild(d.createElement("div"));
    alertObj.id = "alertBox";
    if(d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
    alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth)/2 + "px";
    alertObj.style.visiblity="visible";

    h1 = alertObj.appendChild(d.createElement("h1"));
    h1.appendChild(d.createTextNode(ALERT_TITLE));

    msg = alertObj.appendChild(d.createElement("p"));
    //msg.appendChild(d.createTextNode(txt));
    msg.innerHTML = txt;

    btn = alertObj.appendChild(d.createElement("a"));
    btn.id = "closeBtn";
    btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
    btn.href = "#";
    btn.focus();
    btn.onclick = function() { removeCustomAlert();return false; }

    alertObj.style.display = "block";
    
}

function removeCustomAlert() {
    document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}
</script>

<input type="button" value = "Test the alert" onclick="alert('This is a custom alert dialog that was created by over-riding the window.alert method.');" />
</apex:page>

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