Text contrast sufficient (enhanced)

Myndex rests his argument on Lovie-Kitchin's critical contrast, the point past which further increases in text-to-background contrast produce no improvement in reading speed or comprehension. Once a design reaches that Lc level it enters contrast constancy, and from there the spatial factors — whitespace, stroke weight, size, crowding — govern readability, with the perceived luminance difference ranking, in his words, among the least important. He cites Chung and Tjan for the mechanism: reading speed shows strong spatial-frequency dependence when text contrast is low, which is to say the payoff from added contrast is conditional on size and weight rather than additive to them. sdw32's glance study, which tested APCA Lc 30, 45, 60 and 75, supplies a matching data point, with Open Sans Extra Bold at APCA Lc 30 outperforming Open Sans Light at Lc 75, from which he concludes a single contrast threshold for all font weights is not fit for purpose (the weight-quantification method behind it is covered under text-contrast-sufficient-minimum).

Myndex's #207 restructure groups 1.4.3 Contrast (Minimum) and 1.4.6 Contrast (Enhanced) under one guideline, "Visually readable content uses ample luminance contrast," keeping 1.4.6 as its own row and attaching to it 1.4.6a on font weight and glyph characteristics and 1.4.6b on text use cases. His categories are FLUENT — high-fluent body text, defined as more than two continuous lines in a block or column at an x-height of about 16px or less, fluent text up to two and a half lines, and large fluent headers above about 32px — and SUB-FLUENT, covering soft readability, spot readability (readable with fixation), logo and branding colours, and incidental text in images. GreggVan, in #266 and without reference to the enhanced level, proposes a text visibility tool with one pass threshold X for running text and a different X for menus, buttons and other short strings where the reader has time to fixate.

Answering sdw32's question about minimum size and contrast requirements for default presentation, GreggVan's position is that if a size is large enough for typical vision, then that size under magnification is good enough for low vision — large print for typical viewers is large print for low vision users, fine print is fine print, and anyone with very low vision has magnification built into every OS. He adds that stating any floor would require specifying screen size and CSS pitch, which the author does not control, and says a lot of thought went into concluding there is no way to guarantee text is never too small.

sdw32 rejects the premise of typical vision outright — vision is a bell curve like grip strength, and he routinely measures young people at 20/10, twice the 20/20 reference. Since designers default to designing for themselves, a site built by a 20/10 designer accommodates only 20/50 at the browser maximum of 500% zoom, which he does not accept as maximising foreseeable use. Working backwards from 20/200 access at 500% gives a target of 20/40 suitability at 100%, which is his 8 CSS px x-height floor. On GreggVan's pitch objection he answers that no screen pitch need be specified, only the CSS px definition itself — 96 DPI viewed at 28 inches or 711mm, implying 427mm for a 160 DPI phone — and offers a live failure, the string "achetés = 30% d'économies" on carrefour.fr, as text that fails EN 301 549 while passing the current WCAG 3 draft. The minimum-level half of this exchange is covered under text-contrast-sufficient-minimum and text-style-readable-minimum.

EN 301 549 v4.1.0, which sdw32 worked on, already ships the two-level structure but keys it to whether the text can be enlarged rather than to a conformance level. Zoomable text gets a best-practice x-height of at least 8 CSS px, corresponding to 1.2mm at a 400mm viewing distance with no zooming or screen scaling; text and images of text necessary for all functionality that is closed to text enlargement get at least 16 CSS px of x-height, an exact doubling. The weight rule he wrote for the separate non-digital standard interpreting the European Accessibility Act is two-sided rather than a floor: body text between regular-weight and semi-bold Open Sans, and keywords or short phrases neither thinner than regular Open Sans nor heavier than extra-bold.

APC-RC carries silver and gold levels with reference fonts a designer matches their own typeface against, and Myndex states the selected contrasts for both are intended to serve a Pelli-Robson score down to 1, a profound contrast deficit on a scale where 2 is normal and lower is worse. That makes the higher tier an answer to a measured population rather than a round multiple of the minimum. nakano0949-web pressed him in #590 on whether APC-RC addresses colour vision deficiency and low vision, which she said physical contrast models alone cannot fully capture; Myndex answered that APCA already improves on WCAG 2 for protans, that a protan compensator pre-shaping luminances for colours with high red primary content is due this year, and that SACAM, an accessibility-adjustable colour model for dynamic personalisation, is in progress. In #207 he notes the same physics already rejects pure red against black or dark colours, since red on black is poor for standard vision too.