This is the inner box.

It uses absolute positioning in the CSS to shift its position. Here is the relevant CSS:

div#container {
  position: relative;
  ...
}
div#inner_box {
  ...
  position: absolute;
  left: 10px;
  top: 10px;
}

So you see that, because its parent element has relative positioning applied, this element's position is shifted relative to the top-left corner of its parent element even though it uses absolute positioning, which normally is relative to the top-left of the browser window.