You’ve probably felt it before: you write a neat CSS variable to keep your colors or spacing consistent, only to notice your page feels sluggish during some interactions. Maybe animations stutter, or style recalculations take longer than expected. It’s tempting to assume CSS variables are just a handy, cost-free abstraction, but under the hood they have real runtime costs that can sneak up on you. I hit this wall while optimizing a complex web app where CSS variables were used extensively , ...