D4: Math engine

The design listed MiTeX + typst (A) and ReX (B). ReX has no maintained release: the rex crate on crates.io (4.0.0) is an unrelated programming language. Since the design was written, several pure-Rust TeX math engines have appeared. RaTeX was the only one with meaningful adoption and a release history, so it replaced ReX as candidate B.

Crate	Status on 2026-09-28	Considered
typst 0.15.1 + MiTeX 0.2.7	typst very active (2.8 M downloads); MiTeX's last crates.io release is 0.2.4 (2024-06), the repository is at 0.2.7	Candidate A
RaTeX 0.1.14	Created 2026-03, 22 releases, last commit 2026-09-23, 1,500 GitHub stars, 80 k recent downloads	Candidate B
katex-rs 0.3.0	A KaTeX port that produces HTML and MathML	No: needs a DOM
mathtex, latex-rust, leaf-math, formulary	Weeks old, under 2,000 downloads each	No: too young

100 formulas in 11 groups: basics, fractions and roots, big operators, delimiters, accents and alphabets, matrices, amsmath environments, text and spacing, relations and arrows, braces and decorations, and a mixed group (physics, chemistry, probability). 46 are display formulas, 54 inline. Every formula was rendered by both engines and compared side by side by eye.

Measure	typst + MiTeX	RaTeX
Rendered	88 / 100	98 / 100
Rendered but wrong (visual check)	3: array loses its rules, \tag and equation numbers disappear, \xleftarrow[g]{} prints literal brackets	0 found
Failures	5 from symbol names MiTeX emits but typst 0.15 renamed (\cap, \partial, \hbar, \middle); 7 unsupported: \mathscr, gather*, multline, alignat, \mbox, \color, \ce	multline, \mbox
Start-up (first formula)	11 to 17 ms	1 ms
Layout and SVG per formula, p50 / p90	0.25 / 0.4 ms	0.38 / 0.6 ms (glyphs as outlines)
SVG to pixels at 2×, p50	0.27 ms	0.31 ms
Binary size added (stripped)	+40.1 MB with typst's bundled fonts; about +34.7 MB with only the New Computer Modern fonts it needs	+4.6 MB including the KaTeX fonts (548 KB)
Input language	Typst math: LaTeX must be translated, and the translation lags typst releases	LaTeX as KaTeX accepts it: the syntax Org documents already use
Output	typst frames, SVG	A display list of glyph outlines, rules and paths; SVG, PNG and PDF renderers
License	Apache-2.0; fonts under the GUST Font License	MIT; KaTeX fonts under SIL OFL 1.1

Layout quality is comparable where both succeed. Both follow TeX's rules: fraction bars, script placement, stretchy delimiters and big operators match closely.

End to end in the gpui spike (D3), RaTeX renders a formula to a painted image in 0.2 to 0.5 ms on a background thread. Scrolling through 9,000 distinct formulas keeps 120 Hz.

• Coverage of the syntax people write. Org documents contain LaTeX, and KaTeX's dialect is the de facto standard for LaTeX math outside TeX. Translation to Typst adds a second failure point, and it fails silently in places.
• Size. Typst alone would take most of the 40 MB binary budget (§15). RaTeX costs 4.6 MB.
• Start-up. No library or font set-up at first use.
• Output. The display list can later be painted directly with gpui paths and glyphs, skipping SVG and rasterization, and it maps cleanly to terminal image protocols.

Risk	Mitigation
Young project (6 months), one main author (about 83% of commits)	Pin versions; keep RaTeX behind an org-math trait so an engine can be swapped; the MIT license allows forking. Contribute fixes upstream first (§4.7).
multline and \mbox are missing	Report upstream. Until fixed, \mbox can be mapped to \text and multline to gather in Kalem's pre-processing.
\newcommand from #+LATEX_HEADER (§9.2)	Verified when org-math was built: RaTeX takes macros, but like KaTeX it refuses \newcommand for a name KaTeX already defines (\R, \N), which papers often do. org-math turns every definition into \def, which may redefine.
A formula that fails to render	Show the source with a red frame (§9.2); never hide text.

Typst remains an option for exporting whole documents (a Typst backend or PDF through typst) under D5 and §9.3. That is a separate trade-off with different size and quality considerations, and it would be an optional component.