Large text threshold

Criterion 1.4.3 asks for a contrast ratio of 4.5:1, and then makes one exception: large-scale text needs only 3:1. Large-scale means at least 18 point, or at least 14 point when the text is bold. The reasoning is straightforward, if not quite exact. Bigger letterforms put more pixels of ink on the page per stroke, so the same measured ratio is easier to resolve, and the standard buys a little contrast back in exchange for size. At the enhanced level, 1.4.6, the same split runs one notch tighter: 7:1 normally and 4.5:1 for large text.

The trap is the unit. WCAG states the threshold in points because it was written for a world of printed style guides, and CSS is authored in pixels. The conversion the specification itself uses is 1pt equals 1.333px, so 18pt is 24px and 14pt bold is 18.66px, which nobody rounds the same way twice. Teams routinely convince themselves that a 20px heading is large text. It is not, unless it is bold. Two more details catch people out. Bold means the weight the browser actually renders, so a 700 declaration on a family with no bold face may not earn the discount. And the threshold is about rendered size, so a heading that shrinks at a narrow breakpoint can quietly cross back over the line while nobody is testing that width.

Two things it is not. Non-text contrast is 1.4.11, and its 3:1 applies to icons, borders, and control boundaries at any size, which is a different criterion that happens to use the same number. And APCA, the candidate replacement algorithm, dissolves the whole threshold: it treats size and weight as continuous inputs to one lightness contrast calculation rather than as a switch between two fixed ratios, which is a better model of how reading actually works and is also why an APCA score cannot be compared to a WCAG one.

Practically, this is the number that decides how much of a palette is usable. It is what makes a mid-grey legal in a display heading and illegal in the body copy underneath it, and it is worth knowing before you promise a conformance level on a design that leans on soft greys. It also has to survive dynamic type: text that a reader has scaled up crosses into large-scale territory, which means it can only get safer, while a design that hard codes small sizes to keep a layout intact removes the exception it was relying on.