Although elements like <div>s normally grow to fit their contents, using the float property can cause a startling problem for CSS newbies: if floated elements have non-floated parent elements, the parent will collapse.

floatparent_height

.content:after{
   content: " ";
   display: block; 
   height: 0; 
   clear: both;
   *zoom: expression( this.runtimeStyle['zoom'] = '1', this.innerHTML += '<div class="ie7-clear"></div>' );
}
.ie7-clear {
    display: block;
    clear: both;
}