Your axe run is green and your dark mode has 1.04:1 contrast

I shipped a page that reported zero axe violations . It had button text at a contrast ratio of 1.04:1 — which is, for practical purposes, invisible text. The scan wasn't broken. It was answering a narrower question than I thought I was asking. The bug I had a theme system built the ordinary way. Tokens on :root , overridden in a prefers-color-scheme media query, and overridden again by an explicit [data-theme] attribute so a manual toggle wins in both directions. Buttons came in two flavours: a solid primary and a bordered secondary. .btn { background : var ( --accent ); color : var ( --panel ); } .btn.sec { background : transparent ; color : var ( --ink ); } In dark mode the accent goes light green, so white-on-accent stops working. I patched it the way you patch things at 1am: :root [ data-theme = dark ] .btn { color : #10241b } @media ( prefers-color-scheme : dark ) { :root:not ([ data-theme = light ]) .btn { color : #10241b } } Now count the specificity. Selector Specificity .btn.sec 0,2,0 :root[data-theme=dark] .btn 0,3,0 :root:not([data-theme=light]) .btn 0,3,0 :not() doesn't add specificity of its own, but its argument does. So :root (0,1,0) + [data-theme=light] (0,1,0) + .btn (0,1,0) lands at 0,3,0. My theme patch outranks the component modifier. In dark mode, every secondary button — transparent background, sitting on a #1a1c1f panel — got painted #10241b . Dark green on near-black. 1.04:1. The nasty part is that this class of bug is invisible in review. The rule looks correct. It is correct, for the buttons it was written for. It just also matched buttons it was never meant to touch, in one theme only. Why the scan didn't catch it axe-core evaluates the DOM as currently rendered . It reads computed styles, and computed styles resolve exactly one colour scheme: whichever one the browser is in right now. So npx axe https://example.com is not "does this page pass contrast." It's "does this page pass contrast in the scheme this headless browser happened to boo