The Cascading Style Sheets (CSS) Working Group has published a Last Call Working Draft of CSS Flexible Box Layout Module Level 1. The specification describes a CSS box model optimized for user interface design. In the flex layout model, the children of a flex container can be laid out in any direction, and can “flex” their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent. Both horizontal and vertical alignment of the children can be easily manipulated. Nesting of these boxes (horizontal inside vertical, or vertical inside horizontal) can be used to build layouts in two dimensions. Comments are welcome through 22 April 2014.