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Horn

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What Horn is

Horn is a free, open-source checker for PDF/UA-1 and PDF/UA-2 accessibility documents, built around Matterhorn Protocol 1.1. It automates 87 machine-checkable conditions and surfaces 48 human-judgment conditions for manual review; two conditions have no defined test. Horn reports 100% on the veraPDF PDF/UA-1 corpus. Parallel processing is reported at more than 500 PDFs per second, with the README describing approximately 660 per second in release mode. You can use it through a CLI, desktop application, browser WebAssembly build, npm package, GitHub Action, or Docker. Desktop support covers Linux, macOS, and Windows. Results are available as readable text, JSON, SARIF, or JUnit XML. The GitHub Action can upload SARIF to GitHub Code Scanning, and a GitLab CI example publishes JUnit XML as a test artifact. Developers can add checks through the Check trait. Horn lists zero runtime dependencies and uses the MIT / Apache 2.0 License.

Who it suits

Accessibility teams, developers, and CI maintainers checking PDF/UA documents. It suits workflows that combine automated scans with human review.

What it does well

  • Validates PDF/UA-1 and PDF/UA-2
  • Automates 87 Matterhorn conditions
  • Multiple CLI, desktop, web, and CI options
  • SARIF and JUnit XML outputs
  • Zero runtime dependencies

What to know first

  • 48 conditions still require human judgment
  • Two conditions have no defined test
  • Automated scans are not a complete assessment

Verdict

Horn offers broad PDF accessibility checking with command-line, desktop, browser, and CI delivery options. Its manual-review conditions mean results should be followed by human assessment.

Facts from Horn's own pages

Automated scans
Yes · github.com, 23 Sept 2026
Browser extension
No · github.com, 23 Sept 2026
CI/CD integration
Yes · github.com, 23 Sept 2026
CI/CD output
Horn supports SARIF for GitHub Code Scanning, JUnit XML, and JSON output. · horn.report, 11 Oct 2026
Customization
The site says developers can add custom checks with the Check trait. · horn.report, 11 Oct 2026
Desktop platforms
The README describes Linux, macOS, and Windows support, and the releases page lists binaries and desktop installers for those platforms. · github.com, 12 Oct 2026
Extensibility
The README says custom checks can be added through the Check trait. · github.com, 12 Oct 2026
Free plan
Yes · github.com, 23 Sept 2026
GitHub Action
The site says the GitHub Action supports automatic SARIF upload. · horn.report, 11 Oct 2026
GitHub integration
The GitHub Action can automatically upload SARIF results to GitHub Code Scanning. · github.com, 12 Oct 2026
GitLab integration
The README includes a GitLab CI example that publishes JUnit XML as a test report artifact. · github.com, 12 Oct 2026
License
The project site says Horn is released under the MIT and Apache 2.0 licenses. · horn.report, 12 Oct 2026
Manual review limit
The site identifies 48 human-judgment conditions for manual review and 2 conditions without a defined test. · horn.report, 11 Oct 2026
Manual review tools
Yes · github.com, 23 Sept 2026
Matterhorn checks
For Matterhorn Protocol 1.1, Horn automates 87 machine-checkable conditions and surfaces 48 human-judgment conditions for manual review. · github.com, 12 Oct 2026
Mobile testing
No · github.com, 23 Sept 2026
Output formats
Horn supports human-readable text, JSON, SARIF, and JUnit XML output. · github.com, 12 Oct 2026
PDF standards
Horn validates PDF/UA-1 and PDF/UA-2 documents. · github.com, 12 Oct 2026
PDF/UA coverage
The site says Horn automates 87 machine-checkable Matterhorn Protocol 1.1 conditions and surfaces 48 human-judgment conditions for manual review. · horn.report, 11 Oct 2026
Performance
The project README reports approximately 660 PDFs per second with parallel processing in release mode. · github.com, 12 Oct 2026
Purpose
Horn is an open-source PDF/UA accessibility checker based on the Matterhorn Protocol. · github.com, 12 Oct 2026
Release
The GitHub releases page lists v0.4.0 as the latest release. · github.com, 12 Oct 2026
Runtime dependencies
The site lists 0 runtime dependencies. · horn.report, 11 Oct 2026
Speed
The site says Horn processes 500+ PDFs per second with parallel processing. · horn.report, 11 Oct 2026

Where Horn stands on Freedom251

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