Per w3c/wcag#3642: SC 1.2.1 does not require alternative for time-based media for video-only to be visible.
Remove "visible" from Expectation of these rules:
From ACT TF May 2 discussion -
- Removing "visible", the Expectation would be
The visual information of each test target is available through a text transcript that is included in the accessibility tree, and is either on the page or linked.
- Pass Examples should include visible transcript on page, non-visible transcript on page, linked transcript.
- Failed examples should include visible transcript not included in accessibility tree, and SC 1.3.1 should be a secondary requirement.