The problem happens when a floated element is within a container box, that element does not automatically force the container’s height adjust to the floated element. When an element is floated, its parent no longer contains it because the float is removed from the flow. You can use 2 methods to fix it:

Once you understand what is happening, use the method below to “clearfix” it.

DEMO

Source code for webtoolkit.clearfix.css

.clearfix:after {
        content: ".";
        display: block;
        clear: both;
        visibility: hidden;
        line-height: 0;
        height: 0;
}
 
.clearfix {
        display: inline-block;
}
 
html[xmlns] .clearfix {
        display: block;
}
 
* html .clearfix {
        height: 1%;
}