.ShowMe{display:block;}

function(url)
{
document.getElementById('myIFrame').src = url;
document.getElementById('myIFrame').className = ShowMe;
}

<script type="javascript">
    var iframe = document.createElement("myIFrame");
    iframe.src = url;

        if (navigator.userAgent.indexOf("MSIE") > -1 && !window.opera){
                iframe.onreadystatechange = function(){
            if (iframe.readyState == "complete"){
                //not sure if your code works but it is below for reference
                  document.getElementById('myIFrame').class = ShowMe;
                  //or this which will work
                  //document.getElementById("myIFrame").className = "ShowMe";

                }
            };
        }
        else
        {
            iframe.onload = function(){
                  //not sure if your code works but it is below for reference
                  document.getElementById('myIFrame').class = ShowMe;
                  //or this which will work
                  //document.getElementById("myIFrame").className = "ShowMe";
            };
        }
</script>

window.onload = function () {
    window.frameElement.className = 'ShowMe'; // 'ShowMe' or what ever you have in ShowMe variable.
}