window.onload = function()
{
    var iFrm   = document.getElementById("iFrmCont");
    var doc    = iFrm.contentWindow.document.open("text/html","replace");
    var txt    = "<pre>" + document.getElementById("cont").value + "</pre>";
    doc.write(txt);
    doc.close();

    var styleTag   = doc.createElement("style");
    styleTag.setAttribute("type", "text/css");
    
    try
    {
        var cssText = document.createTextNode("FONT {font-size:100%; line-height:0.8em;}");
        styleTag.appendChild(cssText);
        doc.head.appendChild(styleTag);
    }
    catch (e)
    {
        // IE8
        styleTag.styleSheet.cssText = "FONT {font-size:12px; line-height:1.8em;}";
        var head = doc.getElementsByTagName("head")[0];
        head.appendChild(styleTag);
    }
}

...............

<iframe id="iFrmCont" style="width:100%;height:400px;border:0px" frameborder="0"></iframe>
<textarea id="cont" style="width:0px;height:0px;display:none"><%=post_cont%></textarea>

...............