🦊 FenBrowser

A ground-up, standards-driven web browser engine written entirely in C#.

FenBrowser is an experimental browser engine that implements the web platform from scratch — HTML parsing, CSS cascade & layout, GPU-accelerated rendering, and a custom ECMAScript runtime — all in modern .NET, without wrapping Chromium, WebKit, or Gecko.

Note

This is a research & learning project. FenBrowser is not a daily-driver browser. Many websites won't render correctly (or at all) yet. We're building this to deeply understand the modern web stack, not to replace your current browser.

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✨ Highlights

	Feature	Details
📄	Standards-Based HTML Parser	WHATWG-compliant HTML5 tree builder with incremental streaming, interleaved tokenize/parse for large documents, and conformance guardrails
🎨	Full CSS Pipeline	Tokenizer → Cascade → Computed Style → Used Values. Supports Selectors Level 4 (:has(), nth-child(... of ...), attribute flags), Media Queries Level 4, CSS Color 4 (oklch, oklab), and modern shorthand parsing
📐	Multi-Context Layout Engine	Block, Inline, Flex, Grid, Table, and Float formatting contexts with spec-aligned sizing, placement, and baseline synthesis
🖼️	Skia-Powered Rendering	GPU-accelerated paint pipeline via SkiaSharp + OpenGL, with compositing stability controls, damage-region tracking, partial rasterization, and z-index resolution
⚡	FenJS — Custom ECMAScript Engine	Lexer → Parser → AST → Bytecode compiler → Interpreter, with a logical heap, native intrinsics, and Test262-first verification
🔬	Built-In DevTools	Native element inspector, style panel, debugging overlays, and remote debugging protocol
🤖	WebDriver Protocol	W3C WebDriver support for automated testing and CI integration
🔒	Security-First Design	Centralized navigation/network security policy, process isolation interfaces (in-process + brokered modes), POSIX sandbox launcher support
🌍	Cross-Platform Foundation	Windows primary, with Linux/macOS PAL support (POSIX shared memory, sandbox helpers)

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🏗️ Architecture

┌─────────────────────────────────────────────────────────┐
│                    FenBrowser.Host                       │
│          Windowing · Input · Event Loop · OS Shell       │
├────────────────────────┬────────────────────────────────┤
│   FenBrowser.DevTools  │       FenBrowser.WebDriver     │
│   Inspector · Debug    │       W3C Automation           │
├────────────────────────┴────────────────────────────────┤
│                  FenBrowser.FenEngine                    │
│  HTML Parser · CSS Cascade · Layout · Paint · Scripting │
├─────────────────────────────────────────────────────────┤
│                    FenBrowser.Core                       │
│      DOM · CSS Types · Networking · Security · Logging  │
└─────────────────────────────────────────────────────────┘

Layer	Project	Responsibility
Core	FenBrowser.Core	DOM nodes, CSS value types, HTTP stack, security policy, logging
Engine	FenBrowser.FenEngine	HTML/CSS parsing, style resolution, layout computation, Skia rendering, JS execution
Host	FenBrowser.Host	OS window, input capture, frame timer, navigation lifecycle
DevTools	FenBrowser.DevTools	Native inspector UI, remote debug protocol
WebDriver	FenBrowser.WebDriver	W3C WebDriver endpoint for automation
JS Engine	FenBrowser.Js	Standalone ECMAScript engine (lexer, parser, bytecode, VM)

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📂 Repository Structure

FenBrowser/
├── FenBrowser.Core/            # DOM, networking, CSS types, security primitives
├── FenBrowser.FenEngine/       # Layout engine, CSS cascade, Skia renderer, scripting
├── FenBrowser.Host/            # Desktop shell (entry point, windowing, input)
├── FenBrowser.DevTools/        # Built-in developer tools
├── FenBrowser.WebDriver/       # W3C WebDriver implementation
├── FenBrowser.Js/              # Standalone ECMAScript engine
├── FenBrowser.Js.Shell/        # JS engine CLI (--eval, --dump-ast, --dump-bytecode)
├── FenBrowser.Js.Test262/      # Test262 conformance runner
├── FenBrowser.Tests/           # Unit & integration tests
├── FenBrowser.Conformance/     # Web platform conformance tests
├── FenBrowser.Tooling/         # Harness tooling (Acid2, WPT, debug captures)
├── docs/                       # Canonical technical documentation
├── scripts/                    # Build, test, and CI scripts
└── FenBrowser.sln              # Solution file

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🚀 Getting Started

Prerequisites

• .NET 10 SDK (pinned via global.json)
• Visual Studio 2022+ or any editor with .NET support
• Windows 10/11 (primary), Linux or macOS (experimental)

Build & Run

# Clone the repository
git clone https://github.com/user/FenBrowser.git
cd FenBrowser

# Build the solution
dotnet build FenBrowser.sln

# Run the browser
dotnet run --project FenBrowser.Host/FenBrowser.Host.csproj

Run Tests

# Run the full test suite
dotnet test FenBrowser.Tests/FenBrowser.Tests.csproj

# Run a specific test slice
dotnet test FenBrowser.Tests/FenBrowser.Tests.csproj --filter "FullyQualifiedName~Layout"

Try the JS Engine

# Evaluate an expression
dotnet run --project FenBrowser.Js.Shell/FenBrowser.Js.Shell.csproj -- --eval "1 + 2"

# Dump the AST
dotnet run --project FenBrowser.Js.Shell/FenBrowser.Js.Shell.csproj -- --dump-ast "function hello() { return 42; }"

# Run Test262 subset
dotnet run --project FenBrowser.Js.Test262/FenBrowser.Js.Test262.csproj -- \
  --runtime-subset --root external/test262 --test262 built-ins/Object --max 10

Diagnostic Output

When running, FenBrowser writes rich diagnostic artifacts to logs/:

File	What It Shows
debug_screenshot.png	Raw frame capture of the current render
dom_dump.txt	Full DOM tree and layout box state
fenbrowser_*.log	Module-specific traces (CSS, Layout, Performance)

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🛠️ Tech Stack

Component	Technology	Purpose
Language	C# / .NET 10	Core implementation language
2D Graphics	SkiaSharp 2.88	GPU-accelerated rendering (same engine as Chrome/Android)
Text Shaping	HarfBuzz (via SkiaSharp.HarfBuzz)	Complex script rendering for 100+ writing systems
Text Layout	RichTextKit	Paragraph layout, word wrapping, hit testing
Windowing	Silk.NET	Cross-platform window creation, input, OpenGL context
SVG	Svg.Skia	SVG parsing and rendering
Testing	xUnit	Unit and integration test framework

All runtime dependencies are MIT or Apache 2.0 licensed. Zero telemetry. Zero analytics.

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📋 Standards Compliance

FenBrowser targets conformance with core web standards:

• HTML: WHATWG HTML Living Standard (HTML5 tree builder)
• CSS: Selectors Level 4, CSS Color 4, Media Queries Level 4, Flexbox, Grid, Table
• DOM: Core DOM interfaces, querySelector/querySelectorAll, element state management
• ECMAScript: Test262-driven development — see docs/test262_results.md for current pass rates
• WebDriver: W3C WebDriver protocol
• Rendering: Acid2 conformance testing (currently ~98% pixel similarity)

Compliance is tracked honestly. See docs/COMPLIANCE.md for the full status breakdown.

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📖 Documentation

FenBrowser maintains a canonical documentation set:

Volume	Scope
Volume I — System Manifest	Architecture overview, build notes, roadmap
Volume II — Core	DOM, networking, CSS types
Volume III — FenEngine	Layout, CSS cascade, rendering pipeline
Volume IV — Host	Windowing, input, OS integration
Volume V — DevTools	Inspector, debugging, profiling
Volume VI — Extensions & Verification	WebDriver, testing, conformance

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🤝 Contributing

We welcome contributions! Please read CONTRIBUTING_ENGINE.md for our working contract:

1. Fix one behavior per change — keep scope minimal and local
2. Reproduce first — identify the broken pipeline stage before coding
3. Test everything — add or update focused tests for every behavior change
4. No site-specific hacks — fixes belong in the engine, not per-domain branches
5. Document honestly — partial support is fine; false claims are not

# Minimum verification per change
dotnet build <touched-project>.csproj -v minimal
dotnet test FenBrowser.Tests/FenBrowser.Tests.csproj --filter "FullyQualifiedName~<affected-slice>" -v minimal

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📜 License

FenBrowser is released under the MIT License. See LICENSE for details.

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_{Built with curiosity, caffeine, and a mass of web specifications.}