Yea, the width is a percentage. It's meant to simulate a fluid grid. I'm working on a site right now that has a row of boxes with excerpts of longer articles. I would like them to all have the same height for neatness, but I don't want to set an explicit height because I would like them to resize to contain roughly the same amount of text at different widths.
So the :before pseudoelement on #container has a bunch of _'s and spaces that reflow like any text will. #element has the actual content, and is absolutely positioned to the edges of its parent.
So as the width changes, the text in the :before element reflows and expands #container's size. #element simply follows the size of #container. The :after pseudoelement is just there for decoration.