This is an example of the CSS box model.

Here are two examples of absolutely-positioned divs. When a div is absolutely positioned, it is taken out of the flow of the HTML document. As a result, these divs will overlap content on the page, because they don't honor any content in the page that isn't absolutely-positioned. To test this absolute positioning, try scaling your browser window. You should see that this text moves to fit, while the divs remain in place.

This is an example of an absolutely-positioned div. The parent is the HTML <body>.

This is an example of an absolutely-positioned div. The parent is the the first div, so the offset is based on the first div's upper-left corner.