http://bbcnews23edlc4bcasukhiq2fqujznufzm6cnldakbdgnryzk5h5igqd.onion/accessibility/forproducts/guides/subtitles
In particular, see Annex E of EBU-TT-D regarding quantisation of timing for example if the video can only be presented at a low frame rate, such as in poor network conditions. 5.3 Display subtitles when lips are moving A subtitle (or an explanatory label) should always be on the screen if someone's lips are moving. If a speaker speaks very slowly, then the subtitles will have to be slow, too - even if this means breaking the timing conventions.