# Bez - a generated web engine
Generate a web rendering engine from specs and tests.
## Why
Building a web engine by hand costs hundreds of engineers and many years, so only a few
companies can have one, and only they decide how the web works. Bez generates the engine
instead: from the specifications, with the three shipping browsers (checked against each
other) and WPT as the tests. Once the pipeline exists, each extra engine costs very
little.
## Goals
- **Complete, or tree-shaken.** The default build is a complete engine. For uses
other than a web browser, a build can instead contain only the web features its
content uses: analyse a site or an app, and every feature it never touches is left out
of the binary, not just switched off (`docs/content-scoped-engine.md`).
- **Fast on any device.** A smaller engine does less work and needs less memory.
- **Easy to embed.** A small engine with a clear API, for apps and devices that today ship
all of Chromium or go without.
- **Fastest to update.** When a spec or test changes, the affected code is regenerated and
re-verified, not rewritten by hand.
## How it works
```mermaid
flowchart LR
spec["spec text"] --> model["model writes
many candidates"]
model --> engine["candidate runs
inside the engine"]
browsers["Chromium, Firefox,
WebKit (cached)"] --> check{"same boxes,
same places?"}
engine --> check
check -- no --> model
check -- yes --> code["committed as
ordinary Rust"]
```
- [`docs/generation-loop.md`](docs/generation-loop.md) — how one rule goes from spec text
to committed code.
- [`docs/geometry-verification.md`](docs/geometry-verification.md) — how the engine's
layout is checked against three browsers.
- [`docs/generated-functions.md`](docs/generated-functions.md) — the layout rules generated so far and what each
batch showed.
- [`roadmap.md`](roadmap.md) — the plan, the decisions and the open questions.
## Status
Computed against browser-compat-data 8.0.4 (17259 leaf keys, 79 rules in [`docs/feature-map.json`](docs/feature-map.json)) on 2026-10-20.
| area | generated | hand-written | linked | oracle only | unreached |
|---|---:|---:|---:|---:|---:|
| overall | 0.6% | 0.3% | 0.5% | 5.7% | 93.0% |
| css | 2.5% | 1.1% | 2.0% | 0.0% | 94.4% |
| html | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
| api | 0.0% | 0.0% | 0.0% | 9.9% | 90.1% |
| javascript | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
| svg | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
| webassembly | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
| http | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
| mathml | 0.0% | 0.0% | 0.0% | 0.0% | 100.0% |
See [`docs/dashboard.md`](docs/dashboard.md) for what these statuses mean and how this table is kept in sync.
DOM, style, box tree and fragment tree are hand-written in [`crates/dom`](crates/dom) and
[`crates/layout`](crates/layout) and pass their browser checks
([roadmap.md → "Phase 1 — Engine bootstrap"](roadmap.md#phase-1--engine-bootstrap)).
Nine CSS 2.1 layout rules live in
[`crates/layout/src/generated`](crates/layout/src/generated); eight were written by
models and admitted against the three-browser vote, and block height keeps its
hand-written rule because no model candidate beat it. Together they pass all 227 recipe
cases and all 7 usable WPT normal-flow pages. Which rules the engine actually calls:
[roadmap.md → "Which resolver each generated function runs"](roadmap.md#which-resolver-each-generated-function-runs).
What would prove the thesis:
[roadmap.md → "Proof of concept"](roadmap.md#proof-of-concept).
## Findings so far
- **Three browsers mostly agree, and the third names the odd one out.** 699 of 705
browser-pair comparisons agreed across 235 documents (commit `3ca7323`); all six
disagreements were twelve-deep percentage nesting, and majority voting named Firefox
the outlier every time. [`docs/firefox-app-units.md`](docs/firefox-app-units.md).
- **That Firefox difference is a real web-compat bug.** Gecko rounds lengths to 1/60 px
where Blink and WebKit use 1/64 px, which makes a flex item wrap only in Firefox or
`offsetWidth` differ by a pixel. Reproductions match Mozilla-diagnosed breakage on
Slack, Google Store and Samsung, and Mozilla bug 1719314.
[`experiments/firefox-compat/`](experiments/firefox-compat/).
- **The WPT vote table agrees across most of the suite.** A stable three-engine majority
covers 2,162,676 of 2,282,301 test/subtest keys (94.8%). [`docs/wpt-votes.md`](docs/wpt-votes.md).
- **Script-observable behaviour agrees too.** Trace probes under a fake-media profile
agree on 15 of 18 engine pairs; the three disagreements are a real platform difference.
[docs/dump-format.md → "Trace protocol"](docs/dump-format.md#trace-protocol).
- **Conformance suites are usable oracles.** Counting a test usable when two of three
engines agree: WPT canvas 82.6% (92.7% excluding tentative), Khronos WebGL with dEQP
99.7%, WPT Web Audio 74.4% (85.6% excluding tentative); the WebGPU CTS about 85% for
validation and about 50% for numeric execution.
[`docs/conformance-oracles.md`](docs/conformance-oracles.md), [`docs/webgpu-cts.md`](docs/webgpu-cts.md).
- **Logic programming earns a narrow place.** Margin collapsing written as Datalog agreed
with the browsers on 1195 of 1195 offsets and caught a seeded bug the geometry check
missed in all 227 cases. [`docs/logic-layer.md`](docs/logic-layer.md).
- **The economics hold for a subset of the platform.** About 55–60% of engine-relevant
compat entries have a usable automated oracle and generatable spec prose; roughly 8–18%
have none. [`docs/platform-economics.md`](docs/platform-economics.md),
[`docs/oracle-coverage.md`](docs/oracle-coverage.md).
- **An engine scoped to one site's content is designed**, and its content analyser has
landed. [`docs/content-scoped-engine.md`](docs/content-scoped-engine.md).
## Open questions
What building each rule costs by hand, how wide a feature's generated part should be,
how much of WPT is reachable without JavaScript, and where IDL-generated code ends:
[roadmap.md → "Open questions"](roadmap.md#open-questions). Area-by-area status:
[`docs/platform-areas.md`](docs/platform-areas.md).
## Sources
- browser-specs, every web spec in one repo: https://github.com/w3c/browser-specs
- web-features, the web platform grouped into features: https://github.com/web-platform-dx/web-features/
- browser-compat-data, every piece of the web surface: https://github.com/mdn/browser-compat-data
- WPT, the web platform tests: https://web-platform-tests.org/
- wpt-gen, WPT tests generated from specs: https://github.com/GoogleChromeLabs/wpt-gen
- webref, machine-readable terms from web specs: https://github.com/w3c/webref