<div id="container">
<div id="some_kind_of_popup">
center me
</div>
</div>
You'd need to wrap it in a container. here's the css
#container{
position: fixed;
top: 100px;
width: 100%;
text-align: center;
}
#some_kind_of_popup{
display:inline-block;
width: 90%;
max-width: 900px;
min-height: 300px;
}