css file how to page in the midle, center

div.ex1
{
width: 777px;
padding: 5px;
border: 1px solid #ccc;
margin: 1px;
}
div.ex2
{
width:350px;
float: left;
padding: 0px;
border: 1px solid #ccc;
margin: 1px;
}
div.ex3
{
float: right;
width: 350px;
padding: 0px;
border: 1px solid #ccc;
margin: 1px;
}
div.ex4
{
position:absolute;
top: 90%;
left: 20%;
right: 20%;
width: 777px;
padding: 5px;
border: 1px solid #ccc;
margin: 1px;
}