Free Accessibility Tools Compared

I review software for a living. Not in a sponsored-content, everyone-gets-five-stars kind of way: I mean I sit down with tools and try to understand what they actually do versus what their marketing says they do. For accessibility and web quality tools specifically, I've been doing this for about four years.

Today I want to give you the most honest comparison I can of the four tools people ask me about most: a free browser-based accessibility checker, an open-source accessibility engine, the browser's built-in audit tool, and SCOUTb2. I'll tell you what each is genuinely good at, what each misses, who it's built for, and what it costs. No score inflating, no punishing tools for not doing things they never claimed to do.

Let's go in order of how long they've been around.

The Nonprofit's Accessibility Checker

This checker has been around since 2001. That's longer than most of the web's current conventions have existed. It's a free tool from an accessibility research nonprofit at a university dedicated to web accessibility. The credibility here is real: the nonprofit's research and the checker itself are both taken seriously by accessibility professionals.

The browser extension loads an overlay directly on the page you're viewing. Errors appear as red icons, warnings as yellow icons, structural elements like headings and landmarks as green and blue icons. You can see where issues are in context, which is genuinely helpful for non-technical users who want to understand the impact of a problem rather than just its code-level description.

What the checker covers: alternative text, form labels, color contrast (with a contrast checker), heading structure, link text, ARIA attributes, landmark regions, and more. It's a solid broad-spectrum accessibility scan aligned with WCAG 2.1 guidelines.

What the checker doesn't cover: anything outside accessibility. No SEO, no performance, no broken links, no security headers. It doesn't report Core Web Vitals. It doesn't scan multiple pages. The free version is single-page only and the paid API is designed for programmatic access rather than individual audits.

Best for: designers, content editors, accessibility specialists, clients who need to understand issues visually. Not ideal for developers who want technical specificity or for anyone who needs more than accessibility coverage.

The Open-Source Accessibility Engine

A leading accessibility consultancy is one of the most respected names in digital accessibility consulting. The engine is their open-source rules engine, and the browser extension wraps it. The free version integrates into browser DevTools. There's a paid Pro version that adds guided tests, intelligent guided testing for WCAG items that can't be fully automated, and more detailed reporting.

The free version tests around 80 rules automatically. The descriptions are technical and specific: you get the WCAG success criterion reference, a description of the impact, an explanation of why it matters, and enough context to fix it without additional research. For developers, this is excellent. You can act on a finding without leaving your DevTools panel.

The engine is also widely used as a testing library in automated test suites (popular testing integrations exist and are well-maintained). This means findings in development workflows often use the same rules engine, which creates useful consistency between dev testing and production auditing.

What the engine checks: WCAG 2.1 A and AA compliance, automatically testable subset (the tool itself acknowledges that fully automated testing catches around 30 to 40 percent of all WCAG issues; the rest require human judgment). Same limitation applies to all automated accessibility tools, to be fair.

What the engine doesn't check: anything outside accessibility. No SEO, no performance, no broken links, no security.

Best for: developers and QA engineers who want actionable, technically precise accessibility findings. The guided tests in Pro add meaningful value for organizations doing serious compliance work.

The Browser's Built-In Auditor

The built-in auditor is part of browser DevTools and available as a Node.js module and a CLI tool. It's free, it's maintained by the search engine's team, and it produces a scored report across five categories: Performance, Accessibility, Best Practices, SEO, and PWA.

The Performance category is where the built-in auditor earns its reputation. Core Web Vitals data, Time to Interactive, Total Blocking Time, Cumulative Layout Shift: these are real measurements from a real browser render, and they're presented with clear explanations and prioritized recommendations. For performance debugging, the built-in auditor is excellent.

The Accessibility category is helpful but partial. The built-in auditor uses the same open-source accessibility engine (the same one described above) but runs a subset of it. The resulting score gives you a directional sense of accessibility health but shouldn't be treated as a compliance audit. A page can score 94 and still have keyboard traps or screen-reader issues that the built-in auditor didn't check for.

The SEO category covers the basics: title tags, meta descriptions, mobile viewport, robots.txt, canonical tags, and a few more. It's a starting point rather than a comprehensive SEO audit. No keyword analysis, no backlink data, limited structural checks.

What the built-in auditor misses: broken links, security headers, i18n, multi-page scanning. The audit runs on a single page in a throttled simulated environment, which means your real-user performance may differ from the auditor's scores (and real-user field data is a better source for actual performance metrics).

Best for: developers who want performance diagnostics and a broad health check. Good as a first pass before deeper investigation with specialist tools.

SCOUTb2

SCOUTb2 is newer than the other three (the extension launched in 2024). It's a browser extension that audits a page for accessibility, performance, SEO, broken links, security, and internationalization in a single scan.

I want to be precise about the accessibility coverage: 25+ checks. The actual number of distinct rules is 26. This covers images, form labels, heading structure, keyboard accessibility, color contrast, ARIA attributes, landmark regions, language attributes, and more. It's a solid automated coverage set, comparable in breadth to the accessibility checker and the free tier of the engine.

The differentiator is scope rather than depth in any single category. Here's the honest comparison: the accessibility engine's free tier catches more accessibility issues than SCOUTb2 does (its rules engine is deeper on pure accessibility). The accessibility checker's visual overlay is more useful for non-technical stakeholders trying to understand context. The built-in auditor's performance diagnostics are more detailed for deep performance debugging.

But SCOUTb2 is the only free tool that meaningfully covers all of these at once, plus broken links, security headers, and i18n. Running one SCOUTb2 scan gives you actionable findings across six categories that would otherwise require three separate tools and 20 minutes of tab switching.

The free tier covers single-page audits. No account needed to scan; you need a free account to export reports. PRO adds multi-page scanning (up to 10,000 pages per scan), background scanning, AI-powered insights and fix suggestions (100 per month), scheduled scans, email notifications, 100-report history, and full export formats including PDF, CSV, and JSON.

Best for: anyone who needs a broad health check across multiple categories in a single pass. Particularly useful for freelancers, small agencies, and developers who want to catch the most impactful issues before deciding which specialist tool to bring in for deeper work.

The Comparison Table

Feature	Accessibility Checker (nonprofit)	Accessibility Engine (open-source)	Built-in Browser Auditor	SCOUTb2
Free tier	Yes	Yes	Yes	Yes
Paid tier	API only	Pro available	No	See pricing
Accessibility checks	Yes (visual overlay)	Yes (80+ rules)	Partial (30-40%)	Yes (25+ checks)
Core Web Vitals	No	No	Yes	Yes
SEO audit	No	No	Basic	Yes
Broken link detection	No	No	No	Yes
Security header checks	No	No	Partial	Yes
i18n checks	Partial	Partial	No	Yes
Multi-page scanning	No	No	No	PRO (10K pages)
Audits live rendered DOM	Yes	Yes	Yes	Yes
Works behind login	Yes	Yes	Yes	Yes

My Actual Take

None of these tools is universally the best. They make different tradeoffs and serve different needs.

If you're an accessibility specialist doing deep compliance work, the open-source accessibility engine (free or Pro) is your primary tool. It has the most rigorous rule coverage and the most technically useful output for fixing issues.

If you're a designer or content person trying to understand accessibility in context, the accessibility checker's visual approach is more intuitive and actionable without requiring developer knowledge.

If you're a developer debugging performance, the built-in auditor is irreplaceable. Nothing else gives you that depth of performance analysis for free.

If you need a broad overview of site health across multiple categories before deciding where to focus, SCOUTb2 is the most efficient starting point. One scan, six categories, actionable findings, free for single pages.

The honest reality is that the four tools complement each other more than they compete. I use all of them at different stages of different projects. But if you asked me which one to install first, right now, and run on a site you care about, I'd say SCOUTb2. Not because it goes deepest in any category, but because the breadth of what it catches in a single pass is the most useful starting point for understanding where a site actually stands.

You can decide from there which specialists to call in. First you have to know what you're dealing with.

All four free tiers are worth installing. Start somewhere. The worst audit is the one you didn't run.

Note: Tool capabilities and pricing may change. Information reflects conditions at time of writing. All automated tools detect a subset of possible accessibility issues; none replace comprehensive manual testing with assistive technologies.

Disclaimer: This article is for informational purposes only and does not constitute legal, professional, or compliance advice. SCOUTb2 is an automated scanning tool that helps identify common issues but does not guarantee full compliance with any standard or regulation.