From A List Apart: The Full Feed –CSS Shapes 101:
Rectangles inside other rectangles: this is what our webpages have always been made of. We’ve long tried to break free from their restrictions by using CSS to create geometric shapes, but those shapes have never affected the content inside the shaped element, or how the element is seen by other elements on the page.
The new CSS Shapes specification is changing that. Introduced by Adobe in mid-2012, its goal is to provide web designers with a new way to change how the content flows inside and around arbitrarily complex shapes – something we’ve never been able to do before, not even with JavaScript.”