<style>
div {
    margin-left: auto;
    margin-right: auto;
    position: absolute;
    background-color: green;
    height: 100px;
}
</style>
<div style="height: 500px; width: 500px; background-color: transparent">
 <div style="left: 10px;"></div>
 <div style="right: 10px;"></div>
 <div style="left: 10px; width: 100px"></div>
 <div style="right: 10px; width: 100px"></div>
</div>