It depends on what browsers you need to support. Based on this data <a href="http:&#x2F;&#x2F;caniuse.com&#x2F;#feat=css-variables" rel="nofollow">http:&#x2F;&#x2F;caniuse.com&#x2F;#feat=css-variables</a> I think it will be quite some time before you see this in sites that need broad browser compatibility. Surely there will be (is?) a shim that will come along.