CSS Variables using just CSS and nothing else. You can argue about if this really can be called ”variables”, and as you see, it's actually called ”custom properties” in the spec. But i'm also sure of that everyone will call it ”CSS Variables”.  

Read more: http://gos.si/blog/on-css-variables

This technique is from "CSS Custom Properties for Cascading Variables Module Level 1"  W3C Last Call Working Draft.