What does content: "."; in css do? It appears to act like clear:both and/or display:block by iteself
===

In the standard Magento stylesheet there are a ton of css classes followed by 

    {
    clear: both;
        content:".";
        display: block;
        font-size: 0;
        height: 0;
        line-height: 0;
        overflow: hidden;
        }


To test what it does I deleted the line and on at least one block it moved all of the content up by about 300px and allowed for floats everywhere. This is when I delete JUST the content:"." line.
It almost seems like it's acting as a display:block or clear:both - or working with them.

I have no idea what it does - can someone enlighten me.