For those commenting about SASS having this forever etc, note that this goes a step further:

    :root { --color: blue; }
    div { --color: green; }
    #alert { --color: red; }
    * { color: var(--color); }

You can't accomplish that with SASS today. More info [0].

In fact, this feature isn't even covered with cssnext:

    The current transformation for custom properties aims to provide a future-proof way of using a limited subset (to top-level :root selector) ... [1]

I'm very curious whether or not this paradigm will be accepted as "best practice", or how it will be used.

[0] https://developers.google.com/web/updates/2016/02/css-variab...

[1] http://cssnext.io/features/#custom-properties-var