Suppose you’re designing a webpage where content is in multiple columns, each of thich has a different background. It looks pretty stupid if the columns end in different places, doesn’t it? If you program WordPress themes or similar, it can look REALLY bad. So how can you make sure the columns line up?
If each column has a fixed width, it’s actually pretty easy. Put all of the columns inside a fixed-width container, and set the background for that container to be a repeat-y image that has the appropriate background color for each part of the row. This technique was popularized by Dan Cederholm in 2004, in his article on Faux Columns.
When the columns have variable widths, it’s a bit more difficult. But that’s an issue for another post.