css如何垂直居中对齐

<div class="modal fade" id="dig-nocode" tabindex="-1" role="dialog" aria-labelledby="nocode-alert">
<div style="display:table;width:100%;height:100%">
<div class="modal-dialog" style="vertical-align:middle;display:table-cell;text-align: center;" role="document">
<div class="modal-content modal-control">
<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span id="pos-control" class="glyphicon glyphicon-remove" aria-hidden="true"></span></button>
<div class="modal-body">
<div class="main-button">
Get discount from link
</div>
<div>
<a id="goto-btn"  rel="nofollow" target="_blank">Go to <span></span></a>
</div>
</div>
</div>
</div>
</div>
</div>

如何居中div，如何居中一个浮动元素?

确定容器的宽高 宽500 高 300 的层。
设置层的外边距
.div {
Width:500px ; height:300px;//高度可以不设
Margin: -150px 0 0 -250px;
position:relative;相对定位
background-color:pink;//方便看效果
left:50%;
top:50%;
}