WDD 130: Web Fundamentals

"For a long time, the only reliable cross-browser compatible tools available for creating CSS layouts were features like floats and positioning. These work, but in some ways they're also limiting and frustrating. The following simple layout designs are either difficult or impossible to achieve with such tools in any kind of convenient, flexible way:

• Vertically centering a block of content inside its parent.
• Making all the children of a container take up an equal amount of the available width/height, regardless of how much width/height is available.
• Making all columns in a multiple-column layout adopt the same height even if they contain a different amount of content."

- MDN