Keep text concise and on-screen long enough for a person to read comfortably.
Interestingly, some early Blu-rays still carry "old-fashioned" Line 21 captions. However, these are often only visible if using a composite (analog) output rather than HDMI. Modern Tools for Subtitle Management subtitle 21
These video reviews and tutorials demonstrate how modern software has evolved beyond legacy Line 21 captioning to provide AI-powered, animated subtitles: Keep text concise and on-screen long enough for