What should happen when a viewer presses C?

I build subtitle.im, a Chrome extension for YouTube transcripts and caption controls. What should happen when a viewer presses C halfway through a video?

The viewer’s choice wins for the rest of that viewing. It stops automatic caption changes. Saving a rule for the video or channel takes a separate action, so turning captions on to catch one line won’t affect the next visit.

If your extension automates a setting the site already controls, test the handover. Set an automatic preference, use the site’s own control, then reload. The manual choice should take effect immediately and the saved preference should apply when you come back.

This guide shows how we explain the three caption modes and the manual override to viewers. https://subtitle.im/blog/youtube-caption-control-modes/