Before building Contrast Checker, we ran the numbers on a claim that shows up repeatedly in accessibility circles: that WCAG 2.x's contrast formula overstates readability for darker color combinations.
A clean WCAG pass that isn't actually clean
Gray text (#7d7d7d) on a black background (#000000): WCAG 2.x puts this at 5.10:1 — a comfortable pass for AA on normal text, no asterisks. We ran the same pair through APCA, the perceptual contrast algorithm documented as WCAG 2.x's replacement in the upcoming WCAG 3, and got Lc -33.3 — well under the roughly 60 APCA considers necessary for reliably readable body text. Same two colors, two very different verdicts.
We didn't take APCA's formula on faith either
Before using our own APCA implementation for anything, we checked it against two widely cited reference figures: black text on white should score Lc ≈ 106.0, and white text on black should score Lc ≈ -107.9. Our numbers came out to 106.04 and -107.88 — close enough to trust for the rest of the tool.
Two numbers, not one verdict silently picked for you
WCAG 2.x is still today's actual legal and practical compliance standard, so this tool doesn't ask you to abandon it. It shows both scores side by side, and flags it explicitly whenever WCAG says pass but APCA says weak — so a borderline dark-mode color choice doesn't slip through just because one formula happened to be generous about it.
