There has been a long discussion on the WebAIM forum today on this subject. Most of us came to the conclusion that compliance with SC1.2.5 requires that audio description be provided for all video content that is not adequately explained in the audio track, even if there are not sufficient spaces in the dialog to use for that purpose.

The SC quite clearly states "Audio description is provided for all pre-recorded video content ...". It specifically says "all", without any caveats. That is the normative requirement.

Unfortunately the Understanding document then tries to alter that by claiming that this audio provision is to be done "During existing pauses in dialogue....". The informative document effectively tries to restrict the application of the normative SC, in all cases where videos have large amounts of unexplained visual content with no space in the audio for it.

Not only does that contradict or soften what the SC itself says, but it also blandly ignores the fact that about half of all videos ever made have hardly any spaces in the audio for adding extra. It also ignores that many videos - particularly educational and technical ones - often have detailed diagrams or illustrations that require very long spaces to insert a description. It has also led to widespread confusion in the web dev world. Just last week I was auditing an art museum website, with many videos that displayed a painting or sculpture and then discussed it for about 20 minutes, without one of them ever telling blind people what the painting or sculpture showed! I struggled with explaining this SC to them.

Whether website and video owners choose to comply with the SC for already existing videos, or instead claim undue burden, is up to them. But the SC itself should be clear, without having an Understanding doc appearing to say something different. And it should be a basic requirement of the WCAG at Level AA that all video content be fully accessible to blind people in the audio form without having huge chunks of many videos missing or unexplained or not described to them. That should never have been relegated to AAA in the first place; it's such a basic right!

So to solve this contradiction (not to mention to help ensure that blind people have full access to all video content), could I suggest that a green Note be added to the Understanding doc for 1.2.5. It need not contradict what is already said in the Understanding document. I recommend a wording something like:

Where there is not sufficient space in the existing audio track to insert a particular needed item of audio description, space should be created to allow the insertion. This can be done by freezing the currently displayed video frame for the duration of the audio insertion, or by inserting additional video frames if preferred.

NB: This will, of course, make the AAA SC1.2.7 almost redundant since it says a similar thing. But we should just accept that. Better a redundancy at AAA level (which hardly any website follows anyway), rather than retain this confusing contradiction of 1.2.5.