When a color is set to currentColor via custom property it doesn't work.
No matter whether property is set in a :root or a .selector scope.
But when it is set as a currentColor directly it does work.
I checked it locally and on Browserstack. Edge ver. 17, 16
:root {
--btn-content--color: currentColor;
}
.btn {
color: red;
}
.btn-content {
color: var(--btn-content--color); // doesn't work
}
.btn-content {
color: currentColor; // works
}
You can try it yourself here: https://jsfiddle.net/9gmbfwu4/
I got it ;]