The OP tweet this about it [0], which links to the CSS spec [1]. It seems like a variable defined as initial triggers a different behavior than a traditional "syntax" error.

[0] https://twitter.com/James0x57/status/1283909866915074048

[1] https://www.w3.org/TR/css-variables-1/#invalid-variables

I was playing around with this here: https://jsbin.com/zebipenidi/1/edit and tried editing the keyword to other things.

When I tried other strings (my name, "null", other CSS "words" like opacity) it broke.

I wonder if this is because the initial variable definition needs to be valid css in order to be replaced?