Support is not too shabby: developer.mozilla.org/en-US/docs/W...
That's support for setProperty itself, not support for cash variables, which won't work with setProperty (or at all on ie without polyfilling - and the best polfyills only support initial evaluation)
So, aside from IE, if your browser is less than a year old (2 years, at least, if you exclude Edge), CSS variables work.
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