Freedom251 › Accessibility Testing Software
Horn
What Horn is
Horn is a free, open-source PDF/UA checker based on the Matterhorn Protocol. For Matterhorn Protocol 1.1, it automates 87 machine-checkable conditions, presents 48 conditions for human review, and reports two conditions without a defined test. Horn also validates PDF/UA-2 documents with 37 rules specific to that standard. It can run as a command-line tool, desktop application, browser WebAssembly build, npm package, GitHub Action, or Docker image. Desktop downloads and prebuilt command-line binaries are listed for macOS, Linux, and Windows, while the WebAssembly package supports browser and Node.js use. Reports can be generated as SARIF, JUnit XML, JSON, or readable text. The GitHub Action can upload SARIF results to GitHub Code Scanning, and repository examples cover GitHub Actions and GitLab CI. Developers can add checks through Horn's Check trait. The homepage reports processing more than 500 PDFs per second with parallel processing and a 100% score on the veraPDF PDF/UA-1 corpus. Horn uses the MIT / Apache 2.0 License.
Who it suits
Horn suits developers, accessibility reviewers, and CI/CD teams checking PDF/UA compliance. Its command-line, desktop, browser, Node.js, Docker, and GitHub Action options support varied workflows.
What it does well
- Automates 87 Matterhorn conditions
- Validates PDF/UA-2 with 37 rules
- Runs on major desktop platforms
- Provides SARIF, JSON, JUnit, and text reports
- Supports GitHub Code Scanning uploads
What to know first
- 48 conditions still require human review
- Two conditions have no defined test
- Source installation requires Rust 1.85 or later
- No defined test exists for two conditions
Plans
| Plan | Billed and limits | Price |
|---|---|---|
| Horn | Open-source; MIT / Apache 2.0 License | Free |
As horn.report lists them, read 13 Oct 2026. The maker's own page has the final word.
Verdict
Horn offers broad PDF/UA checking through free open-source tooling and several deployment formats. It combines automated scans with explicit manual-review reporting and CI/CD outputs for accessibility workflows.
Facts from Horn's own pages
- Browser and Node
- The getting-started guide says Horn can run in a browser or Node.js through its WebAssembly npm package. · horn.report, 13 Oct 2026
- Browser and Node.js
- The getting-started guide says Horn can be used in a browser or Node.js through its WebAssembly npm package. · horn.report, 12 Oct 2026
- CI/CD
- Horn provides a GitHub Action that can automatically upload SARIF results to GitHub Code Scanning. · github.com, 13 Oct 2026
- CI output
- Horn supports SARIF, JUnit XML, and JSON output for CI/CD workflows. · horn.report, 12 Oct 2026
- CLI platforms
- The getting-started guide lists prebuilt CLI binaries for macOS, Linux, and Windows. · horn.report, 12 Oct 2026
- Company details
- The homepage credits Horn Contributors and gives copyright as 2025-present; it does not state a headquarters or founding date. · horn.report, 13 Oct 2026
- Custom checks
- The homepage says developers can add custom checks using Horn's Check trait. · horn.report, 13 Oct 2026
- Deployment
- The getting-started guide documents installation from source and Docker usage. · horn.report, 13 Oct 2026
- Desktop platforms
- The getting-started guide lists desktop downloads for macOS, Windows, and Linux. · horn.report, 12 Oct 2026
- Docker
- The getting-started guide provides instructions to build and run Horn in Docker. · horn.report, 12 Oct 2026
- Extensibility
- The homepage says developers can add custom checks with Horn's Check trait. · horn.report, 12 Oct 2026
- GitHub Action
- The GitHub Action can automatically upload SARIF results to GitHub Code Scanning. · github.com, 12 Oct 2026
- Installation requirement
- Installing Horn from source requires Rust 1.85 or later and edition 2024. · horn.report, 12 Oct 2026
- Integrations
- The repository documents GitHub Actions and GitLab CI examples, as well as SARIF output for GitHub Code Scanning. · github.com, 13 Oct 2026
- License
- The homepage states that Horn is released under the MIT / Apache 2.0 License. · horn.report, 13 Oct 2026
- PDF/UA-2
- The product repository says Horn also validates PDF/UA-2 documents and applies 37 PDF/UA-2-specific rules. · github.com, 13 Oct 2026
- Performance
- The homepage states that Horn processes 500+ PDFs per second with parallel processing. · horn.report, 13 Oct 2026
- Platforms
- The getting-started guide lists pre-built binaries for macOS, Linux, and Windows, plus a desktop app for those systems. · horn.report, 13 Oct 2026
- Product
- Horn is an open-source PDF/UA checker based on the Matterhorn Protocol. · horn.report, 12 Oct 2026
- Purpose
- Horn is an open-source PDF/UA checker based on the Matterhorn Protocol. · horn.report, 13 Oct 2026
- Reports
- Horn supports SARIF, JUnit XML, JSON, and human-readable text output. · github.com, 13 Oct 2026
- Security and compliance
- The homepage describes Horn as an open-source checker for PDF/UA against ISO 14289-1 and says it scores 100% on the veraPDF PDF/UA-1 corpus. · horn.report, 13 Oct 2026
- Standards
- It automates 87 machine-checkable Matterhorn Protocol 1.1 conditions, surfaces 48 human-judgment conditions for manual review, and identifies 2 conditions without a defined test. · horn.report, 13 Oct 2026
- Ways to use
- Horn is available as a CLI, desktop app, browser WebAssembly build, npm package, GitHub Action, and Docker image. · horn.report, 12 Oct 2026
Where Horn stands on Freedom251
- Accessibility Testing Software#66 of 169