Add CSS variable to browser

Documentation at
https://developer.mozilla.org/en-US/docs/Web/API/CSS

Specified at
http://dev.w3.org/csswg/css-conditional/#the-css-interface
http://dev.w3.org/csswg/cssom/#the-css.escape%28%29-method
Merge pull request #1899 from Rob--W/variable-CSS

Add CSS variable to browser