CSS variables have ~96% coverage at this moment: https://caniuse.com/css-variables. If you are not using Internet Explorer, you should very likely be able to render a page using var.
The trouble with caniuse is that it only covers the "mainstream".
It does not cover more exotic things like browser in RiscOS or Amiga or text-only browsers etc. These are often consumers of text-only sites, deliberably or not. Small market share though I admit, but probably larger market share for text-only sites Vs other "normal" sites.
I think covering the "mainstream" is the point of the site. Don't get me wrong, I really do enjoy fast loading text based websites, but if web devs had to worry about those browsers, then CSS (and web technology in general) could never develop past its infancy. Whether that's a good thing or bad thing is up to you.
Almost exactly the same things were the de facto standards when "tabs" were all the rage way back when.
Kudos though for the modern CSS, although things like var etc wont work in old browsers