APCA

APCA reports one number, Lc, on a scale running to about 106 for black on white and about minus 108 for white on black. The sign is the point: the algorithm knows which of the pair is the text, so light text on a dark field and dark text on a light field are two different measurements rather than one. The contrast ratio in WCAG 2 is symmetric by construction, dividing the lighter relative luminance by the darker, and cannot tell those two situations apart at all.

The second departure is that a score alone is not a verdict. APCA pairs its output with a lookup table of font sizes and weights, so the same pair of colours can be fine for a 24px bold heading and much too weak for 14px body text. Roughly, thin small text is asked for around Lc 90, ordinary body text for Lc 75, and large or bold text for Lc 60, with anything under Lc 30 treated as invisible. That reframes contrast as a property of the typography as well as of the palette, which is what practitioners had been doing informally for years.

The disagreements with WCAG 2 are where the argument lives, and they run both ways. Mid grey text on black can pass WCAG 2 at AAA and score under Lc 50, because the old formula’s handling of dark backgrounds flatters them; white on a mid grey can fail 4.5:1 and score comfortably above Lc 75. Neither result is a bug in the tool that produced it. They are two models of the same phenomenon, one fitted to 1990s CRT measurements and one to more recent readability research, and where they part company is usually worth looking at with your own eyes.

Status matters here, so state it plainly: APCA is a candidate for WCAG 3, and WCAG 3 is a working draft with no date. Nothing about APCA is normative, audits and procurement still quote 4.5:1, and a page that satisfies APCA is not thereby conformant. The sane position is to keep meeting the WCAG 2 thresholds you are actually held to and use Lc as the second opinion that catches what a symmetric ratio cannot, particularly in dark themes. Some design systems already publish both: Radix guarantees its text steps to Lc 60 and Lc 90 against a step 2 background, which is a contrast promise written into the colour ramp rather than checked afterwards.