Accuracy

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What “accuracy” means

Accuracy measures whether Pretext’s predicted height for a text block matches the real DOM height that a browser would produce for the same text, font, and container width. For each test case, Pretext runs prepare() followed by layout() and compares the resulting height against the height produced by the browser’s own layout engine for an equivalent DOM element. A case is correct when the heights agree within the line-fit tolerance.

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Current status

These snapshots were captured on 2026-03-27.

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The test corpus

The browser sweep uses a 4-font × 8-size × 8-width × 30-text corpus, producing 7,680 test cases per browser.

• 4 fonts — a representative set of named fonts (not system-ui)
• 8 sizes — spanning typical UI text sizes
• 8 widths — from narrow to wide container widths
• 30 texts — a mix of scripts, emoji, mixed-bidi, and edge-case strings drawn from src/test-data.ts

Each combination is an independent case. The sweep runs in the browser against the live DOM, so it captures actual font rendering, not approximations.

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Line-fit tolerance

Not every height difference represents a real error. Browsers accumulate floating-point rounding across line widths, and the threshold for whether a word fits on a line varies slightly by engine. Pretext uses engine-specific tolerances:

• Chromium / Gecko: 0.005 (pixels)
• Safari / WebKit: 1/64 (pixels)

A height prediction is considered correct if the line-count decision it produces matches the browser’s line count within these tolerances.

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Emoji correction

Canvas measureText reports emoji wider than the DOM renders them at font sizes below 24px on macOS (Apple Color Emoji). The inflation is constant per emoji grapheme at a given size and is font-independent. Pretext auto-detects this correction by comparing canvas vs. DOM emoji width once per font, caches the result, and applies it to all subsequent emoji measurements for that font. Safari canvas and DOM agree on emoji width (both report wider than fontSize), so the correction value is 0 on Safari.

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system-ui caveat

system-ui is not safe to use with Pretext for accuracy-sensitive work on macOS. The canvas context and the DOM can resolve system-ui to different optical variants of the system font, producing systematically different width measurements. Use a named font (e.g. Inter, Helvetica, -apple-system) to guarantee that canvas and DOM measure the same glyphs.

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Refreshing snapshots

Refresh accuracy/chrome.json, accuracy/safari.json, and accuracy/firefox.json when a change affects the browser sweep methodology or the main text engine behavior (src/analysis.ts, src/measurement.ts, src/line-break.ts, src/layout.ts, src/bidi.ts, or pages/accuracy.ts).