始终在屏幕中间显示Div的代码(css+js)

.ordersearchDivCss
{
position: absolute;
z-index: 100;
display: block;
background-color: #6ec1df;
}
<div class="ordersearchDivCss" id="DivMain" style="width: 400px; height:200px" align="center"></div>

function sc1(DivId) {
var Div = $(DivId);
$(DivId).style.top = (document.documentElement.scrollTop + (document.documentElement.clientHeight - $(DivId).offsetHeight) / 2) + "px";
$(DivId).style.left = (document.documentElement.scrollLeft + (document.documentElement.clientWidth - $(DivId).offsetWidth) / 2) + "px";
//alert($(DivId).style.top);
}

function scall() {
sc1("DivMain");
}
window.onscroll = scall;
window.onresize = scall;
window.onload = scall;