CSS Variables allow authors to create reusable values which can be used throughout a CSS document.
For example, it's common in CSS to reuse a single color throughout a document. Prior to CSS Variables this would mean reusing the same color value many times throughout a document. With CSS Variables the color value can be assigned to a variable and referenced in multiple places. This makes changing values easier and is more semantic than using traditional CSS values.
CSS Variables are currently considered an experimental technology.
Firefox: Version 31+ (Enabled by default)
Chrome: Version 49+ (Enabled by default).
"This feature can be enabled in Chrome Version 48 for testing by enabling the
experimental Web Platform feature. Enter
chrome://flags/ in your Chrome address bar to access this setting."
IE: Not Supported.
Edge: Under Development
Safari: Version 9.1+