The master is only the starting point
Open captions are part of the visible presentation rather than a track the viewer can hide. W3C WAI distinguishes that behaviour from closed captions. If the text is burned into picture pixels, review the actual encoded outputs that readers receive.
An invented production has clear captions on its editing master, but a much smaller delivery rendition. The successful master review does not establish the smaller output's readability. Text edges, line wrapping already fixed in the picture and the surrounding scene all need attention.
Review at the intended size
Inspect the demanding caption passages at their intended display size. Include a busy scene, the longest line and a lower-third collision where present. Enlarging a small rendition to fill an editing monitor can hide the practical size at which a reader must interpret it.
MDN's Video concepts guide, by Mozilla Contributors under CC BY-SA 2.5 or later, explains video-image and compression concepts. The release judgement here remains an editorial observation to make on the actual outputs; a codec name or resolution label cannot approve readability by itself.
Keep the caption source editable
Retain the timed-text master alongside the picture master. If fixed captions need revision, an editable source is more useful than attempting to reconstruct words from compressed pixels. Record which language and caption revision were burned into each deliverable.
Consider whether a selectable text track better meets the publication's needs, and check its actual delivery support separately. This article does not declare one method universally superior or claim a performed encode or readability test.
Questions answered
Does legible text on the master prove every rendition is legible?
No. Review the final outputs at their intended presentation sizes.
Can viewers resize burned-in caption text independently?
Text fixed in the picture does not behave like a separately rendered caption track.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.