Constraint Cascading Style Sheets (CCSS), an extension to Cascading Style Sheets (CSS).
Constraint Cascading Style Sheets (CCSS), is an extension to Cascading Style Sheets (CSS). CCSS adds support for layout constraints. These allow designers to describe the layout of a web page in a more flexible manner.
Original CCSS proposal by Badros and Borning (1999): http://constraints.cs.washington.edu/web/ccss-uwtr.pdf
For a current (2015) implementation see Grid Style Sheets at: https://github.com/gss/engine