Which is not obvious at all if you have JavaScript disabled by default, since it only shows up as a blank space, which could also be a blocked ad or an image which failed to load correctly.
The first few times I saw one of these transcripts with video at the top (IIRC, it was on Practical Engineering, not this site), I thought it sounded odd but didn't get that it was a transcript. Only later did I find out that there were videos (and they're great).