conformance #

Where the benchmarks measure performance on real-world code, this page compares one aspect of parser correctness — which files each parser accepts — on edge-case test suites.

Coverage
#

Parsing 4,643 Svelte files

source	files	rsvelte	svelte/compiler	tsv
Svelte's tests	4,243	100.00%	100% by construction	100.00%
prettier-plugin-svelte's tests	318	100.00%	100% by construction	100.00%
Prettier's HTML fixtures	82	100.00%	100% by construction	100.00%

Parsing 53,323 TypeScript/JS files

source	files	tsv	oxc-parser	yuku-parser (wasm)	tsc	swc	acorn-typescript
all sources but TypeScript compiler's cases	45,226	99.94% −26	99.90% −44	99.87% −55	99.83% −76	99.19% −366	99.15% −384
test262	43,739	100.00%	99.96% −14	99.90% −41	99.85% −62	99.27% −316	99.40% −261
TypeScript compiler's cases	8,097	99.01% −80	98.78% −98	98.69% −106	100% by construction	98.01% −161	95.98% −325
Prettier's JS fixtures	756	97.88% −16	98.94% −8	99.33% −5	98.28% −13	97.35% −20	93.78% −47
Prettier's TypeScript fixtures	591	98.30% −10	96.27% −22	98.47% −9	99.83% −1	94.92% −30	87.14% −76
Svelte's tests	140	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%

Parsing 22,450 CSS files

source	files	PostCSS	tsv	svelte/compiler
all sources	22,450	99.60% −88	99.31% −154	99.07% −207
web-platform-tests CSS	22,127	99.60% −88	99.38% −137	99.15% −186
Svelte's tests	184	100.00%	100.00%	100.00%
Prettier's CSS fixtures	139	100.00%	87.76% −17	84.89% −21

Notes:

A parser accepts a file when it reports no error, meaning it doesn't throw or, for the error-recovering parsers (oxc-parser, tsc, yuku-parser), returns no error diagnostic. The dimmed −n beside a percentage is the number of files rejected. A dimmed 100% marks a parser that was used to select the source's files, so its 100% is by construction, not a result: svelte/compiler on the Svelte set and tsc on the TypeScript compiler's cases. The leading row leaves such a source out, so every parser there is measured on the same files and none by construction.
Coverage has two blind spots. Accepting a file says nothing about producing the right AST; tsv's output is verified separately against the parsers it's a drop-in for (Svelte's parser, its parseCss, and acorn-typescript) at corpus scale in its repo's conformance gates, and what it rejects among the TypeScript compiler's cases is catalogued in its tsc notes. And coverage doesn't reward rejecting what should be rejected, so a permissive parser scores well here. tsv is permissive in this sense: it doesn't yet check most early errors, and rejects fewer than half of test262's should-reject parse tests (tracked in its test262 notes).
No CSS parser here is a validity oracle, so the CSS sources keep invalid and out-of-scope inputs (wpt's deliberately invalid CSS, the preprocessor syntax in Prettier's .css fixtures): read a row's parsers against each other, not against 100%. tsv, a drop-in for Svelte's parseCss, sits above it on balance: it parses spec-valid CSS that parseCss rejects, and rejects a few malformed files parseCss lets through. CSS Syntax never rejects a stylesheet, so a spec-compliant parser would recover from those; tsv doesn't yet. PostCSS leads by not parsing selectors, at-rule preludes, or values, which admits preprocessor syntax, much of wpt's deliberately invalid CSS, and some valid modern CSS that tsv and parseCss both reject.
yuku-parser's column is its wasm binding, since the native one segfaults on some of test262's escaped-identifier tests.

versions

svelte 5.57.2
acorn 8.19.0
@sveltejs/acorn-typescript 1.0.13
oxc-parser 0.153.0
@yuku-core/wasm 0.17.0
postcss 8.5.29
@rsvelte/vite-plugin-svelte-native 0.3.17
rsvelte's upstream svelte 5.57.1
@swc/core 1.16.13
typescript 6.0.3

run

October 10, 2026
tsv 0.6.0
c93c164

environment

AMD Ryzen 5 PRO 7530U with Radeon Graphics
linux/x86_64
node 24.14.1

Test corpus
#

80,416 files from 9 sources, all of them formatter, compiler, and conformance test suites. Each source links its upstream, at the commit the harness pinned where the report records one.

The Svelte set is every .svelte and .html file, minus the harness's fixture exclusions and those svelte/compiler rejects. The .html files load as Svelte.
test262 is its expected-valid tests outside test/staging/, minus the sloppy-mode tests under test/annexB/: Annex B is the web-browser layer, optional outside browsers, and tsv doesn't implement it. Over half the set is runtime-library tests (built-ins, intl402) with ordinary syntax.
The TypeScript compiler's cases are the single-file .ts ones tsc's parser accepts and whose recorded baselines carry no grammar error.
The web-platform-tests CSS is the <style> blocks of the .html pages under its css/ directory, deduplicated.
From Prettier's suites the harness drops range- and cursor-marker files, front-matter and multiparser fixtures, Babel-only proposals, error cases its specs expect every spec-grammar parser to reject, and the spec files themselves.
JSX is excluded: Prettier's JSX suite, every .tsx file, and the .js fixtures Prettier reads as JSX are dropped, since every parser here is run in plain TypeScript mode, where all of them reject JSX. tsv has no JSX mode by design, so these tables don't measure that gap.
TS/JS files parse as modules, with two exceptions: test262's parse as script or module, as each test's flags declare, and Prettier's JS and TypeScript fixtures take the goal their extension names (.cjs, .mjs, …), otherwise retrying as scripts when the module parse fails. tsc decides for itself.

source	commit	files	Svelte	TypeScript/JS	CSS
all sources		80,416	4,643	53,323	22,450
prettier-plugin-svelte's tests	`7809486`	318	318	0	0
Prettier's TypeScript fixtures	`1dcd0b0`	591	0	591	0
Prettier's JS fixtures	`1dcd0b0`	756	0	756	0
Prettier's CSS fixtures	`1dcd0b0`	139	0	0	139
Prettier's HTML fixtures	`1dcd0b0`	82	82	0	0
Svelte's tests	`10fdca7`	4,567	4,243	140	184
web-platform-tests CSS		22,127	0	0	22,127
test262		43,739	0	43,739	0
TypeScript compiler's cases	`637d574`	8,097	0	8,097	0