Caption live audio with a captioning service
G157 brings a real-time captioning service onto the page itself. A trained operator listens and types on a stenographic keyboard with only a small delay. They capture speech at high fidelity and add notes on any non-speech audio that matters. The caption viewport sits on the same page as the audio, so nobody is chasing a second window. W3C lists this as sufficient for 1.2.9 Audio-only (Live), which is Level AAA. Be careful with the neighboring rule. Live captions for synchronized media, meaning live video with sound, is a separate Level AA criterion with its own technique set, and G157 is not on it. This one belongs to live audio-only.
How we find it in an audit
Our reviewers join a live event and watch the captions as they are produced. Automated captioning is where this usually falls down. We check accuracy on names and numbers, look for speaker identification, and measure how far behind the text runs. Then we operate the caption region by keyboard and with a screen reader, because a caption viewport nobody can reach is not a caption.
How affected users experience it
A live webinar with no captions is a slideshow with an inaudible soundtrack for deaf and hard of hearing attendees. They can see the demo and get none of the explanation, and they cannot ask a question about a sentence they did not receive. Real-time captions put them in the same room as everybody else, at the same moment, which is the whole point of attending live.
Passes vs. fails
Passes
A live captioning service transcribes the audio as it happens, and the captions render in a viewport beside the stream with speakers identified.
Fails
A live product-launch webinar streams speech and demos with no captions of any kind.
Other ways to satisfy this rule
3 guides on this site are filed under 1.2.9 Audio-only (Live). W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G157: Incorporating a live audio captioning service into a web page. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.