css div水平垂直居中

<STYLETYPE="text/css">...
.middle-demo-4{...}{
height:400;
width:100%;
position:relative;

background:#aaa;
}
.middle-demo-4div{...}{
position:absolute;
top:50%;
left:50%;
background:blue
}
.middle-demo-4divdiv{...}{
position:relative;
top:-50%;
left:-50%;
background:yellow;
overflow:hidden;

}
</STYLE>

<divclass=middle-demo-4>
<div>
<div>helpipsumdolorsitamet,consectetueradipiscingelit.asdsadfs.<br><imgsrc=1.jpgwidth=140height=100>asdsadfs.<br>asdsadfs.asdsadfs.<br>asdsadfs.asdsadfs.</div>
</div>
</div>