Accessibility use case: using Pretext for WCAG text measurement

Hi @chenglou — congratulations on Pretext, it's an incredible piece of engineering.

I'm building **[ReadCheck](https://github.com/rafaelwabisabi-maker/readcheck)**, an open-source accessibility measurement engine that uses Pretext's canvas-based text measurement to detect accessibility barriers on websites — with particular focus on elderly and low-literacy users.

## Why Pretext matters for accessibility

Current accessibility tools (axe-core, Lighthouse, Pa11y) are limited to DOM-attribute checks. They cannot express rules that require **text layout measurement** because `getBoundingClientRect()` triggers expensive reflows. Pretext's `prepare()`/`layout()` architecture changes this — for the first time, we can measure text dimensions at scale without blocking the render pipeline.

This enables **3 novel accessibility rule types** that no existing tool can express:

1. **text-overflow-risk** — predict text overflow at different zoom levels *before* rendering (WCAG 1.4.4)
2. **visual-truncation-unlabeled** — detect `text-overflow: ellipsis` where truncated content isn't exposed via `aria-label` or `title`
3. **line-length-density** — measure characters per visual line for cognitive load scoring (WCAG 1.4.8, anticipating WCAG 3.0 cognitive criteria)

## Context

- 94.8% of the top 1M websites fail WCAG (WebAIM 2025)
- The European Accessibility Act (enforced June 2025) carries €3M fines
- Commercial overlays (accessiBe) were fined $1M by the FTC — the community needs open-source alternatives
- ReadCheck is applying for NLnet NGI Zero Commons Fund to develop this further

## What I'd love to discuss

- Are there accessibility-relevant measurement utilities that would make sense as Pretext built-ins or companion helpers?
- Any guidance on the canvas-to-rendered accuracy gap (sub-pixel hinting, font fallback chains) — we're building a calibration coefficient approach
- Would you be open to a "building accessibility tools with Pretext" documentation contribution?

Early demo: open `demo/index.html` in the ReadCheck repo.

Thanks for building something that makes genuinely new accessibility tooling possible.

Start by reviewing the accessibility use cases and the early demo at demo/index.html in the ReadCheck repository. The issue does not identify a Pretext file, test, or entry point; clarify whether the intended outcome is built-in measurement utilities, companion helpers, or documentation before implementation, then define the accepted scope and validation approach.

Written by the indexing model from the issue text.