How to check captions against WCAG 2.1 AA

“Are these captions WCAG compliant?” is not quite answerable as asked, and the gap between what people mean and what a file can establish is where most caption compliance claims go wrong. This page is about running the check honestly: per criterion, with the criteria that cannot be settled named rather than quietly dropped.

The criteria a caption file bears on

WCAG 2.1 Level AA contains a great deal that has nothing to do with captions. For a caption track it comes down to these, and to how much of each a file can settle:

Cited from W3C Web Content Accessibility Guidelines 2.1, Level AA (W3C Recommendation, 5 June 2018) — read the standard.

Running the check

The thing that makes a result citable is that the standard was named on the request, so the verdict comes back per criterion rather than as a score. In CaptionPass that is a standard selection in the project workbench, or a standard field on POST /api/v1/process for a scripted check in CI.

The anonymous converter on the home page does not take a standard — it grades against a delivery preset, which answers the different question of whether a platform will accept the file. Both are useful; only one of them produces evidence.

The check is deterministic. The same file gives the same verdict, and every finding points at the cue that caused it, which is what lets somebody argue with the result instead of having to trust it.

What a clean result establishes

Less than it looks like, and saying so is the difference between evidence and marketing. For a criterion the file can only ever disprove, no findings means nothing detectable is wrong — not that the criterion is met.

WCAG conformance is also a property of the delivered experience: the player, the page, and the audio together. A caption file is one input. What a check establishes is what that input was tested against and what it found — which is precisely what a VPAT caption row needs, and precisely what an assurance does not provide.

Related reading

For authoring practice — reading speed, sound tags, contrast on burned-in captions — see WCAG-minded captions. That guide is about making captions good; this one is about establishing what they were checked against. If you are here because of a deadline, start with evidencing ADA Title II captions.

What this is not. A conformance receipt is evidence of what was checked, by which engine, against which version of which standard, and what it found. It is not a legal opinion, an audit, or a safe harbour. CaptionPass reads the caption file and never the audio, so anything that needs the audio — above all whether the captions match what was said — is reported as not evaluated rather than assumed.