Free accessibility checker: what automation catches, and what it cannot
Automated testing finds roughly a third of WCAG failures. Knowing which third — and which two thirds still need a human — is the difference between compliance and a clean report.
By Sapun Lamichhane · Arcetis
Automated accessibility testing is genuinely useful and routinely oversold. The honest framing is that it catches about a third of WCAG failures, and that third is worth catching because the fixes are mechanical and the impact is immediate.
The dangerous part is what a clean report implies. A site can pass every automated check and remain unusable.
What automation reliably catches
- Colour contrast below 4.5:1 for body text, or 3:1 for large text. Purely computational, and one of the most common failures on well-designed sites.
- Images with no alt attribute at all. Note that alt="" is correct for decorative images — an empty attribute is a decision, a missing one is an omission.
- Form inputs with no associated label. A placeholder is not a label; it vanishes the moment someone types.
- Interactive elements with no accessible name — the icon-only button being the classic case.
- A missing lang attribute, which leaves a screen reader guessing at pronunciation.
- Duplicate ids, which silently break the label associations above.
What it cannot catch
Everything requiring judgement, which is where most real barriers live.
- Whether alt text is meaningful. "image123.jpg" passes the automated check and helps nobody.
- Whether focus order follows the visual order. A keyboard user tabbing through a page in a nonsensical sequence is a serious failure that no scanner detects.
- Whether an error message explains how to correct the error.
- Whether a custom widget behaves the way its ARIA role promises. Incorrect ARIA is worse than none, and only a human using it can tell.
- Whether the page is usable at 200% zoom, or with a screen reader end to end.
Why the rendered page is the only fair test
Accessibility problems concentrate in exactly the parts of a page that JavaScript builds — modals, menus, custom form controls, anything with a managed focus state. A checker that validates the server's HTML response has not tested any of it.
The overlap nobody mentions
Every fix in the first list also improves how AI agents handle your site, because an agent reads the same accessibility tree a screen reader does. A properly labelled button is operable by an assistant acting for a user for precisely the reason it is operable by someone using a screen reader. The work counts twice.
What free means here
Our accessibility checker is free, with no account needed for your first scan. It runs two independent rule engines against the rendered page and reports which WCAG success criterion each finding maps to — and it says plainly that the result is a floor, not a certificate.