This is currently working in stable Safari but seems to have regressed: https://jsbin.com/metobigecu/1/edit?css,output
Expected: Background is Green
We discovered this while working on a demo for Microsoft Edge custom properties. FF fails due to no calc support in color functions and Chrome doesn't support float values in Calc(). Webkit seems to have the same issue for calc() but doesn't if you're using a variable. At any rate, would be great to see this fixed.
We brought consistency to this, so we fail whether you use a variable or not.
Created attachment 308413 [details]
Patch for EWS
Created attachment 309046 [details]
Landed in r216188.