GSS engine 
Compiles and runs Grid Style Sheet (GSS) rules. GSS is an implementation of Badros & Borning's Constraint Cascading Style Sheets, enabling far better layout control through building relational rules between different elements.
GSS supports the following syntaxes for defining layout rules:
- CCSS - direct constraints related to position and size of DOM elements
- VFL - horizontal and vertical spacing constraints based on Apple's Visual Format Language
Please refer to http://gridstylesheets.org/ for documentation and usage instructions.