Automated accessibility scans can miss contrast failures hidden in dark mode

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A developer discovered that a webpage reporting zero axe accessibility violations contained button text with a near-invisible contrast ratio of 1.04:1 — but only in dark mode. The flaw stemmed from a CSS specificity conflict where a late-night dark-mode patch unintentionally overrode secondary button colours, rendering dark green text on a near-black background. The problem went undetected because axe-core evaluates contrast only in the colour scheme active at scan time, meaning CI pipelines running in light mode will not catch dark-mode contrast failures. Running the same axe scan twice on identical markup — once per theme — confirmed zero violations in light mode versus two critical failures in dark mode. The fix involved replacing direct colour overrides with theme-aware CSS tokens, eliminating the specificity contest and ensuring correct contrast across all themes.

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