If a picture is worth a thousand words, a video is worth a million, and if your web site has a few videos, how many of those millions of words are contributing to your site’s SEO? The sad answer for most content managers is “zero”. Though search engine technology is advancing day-by-day, content indexers cannot reliably process audio into a text equivalent (YouTube does have an experimental closed caption feature, but the results are not great).
Adding a set of text captions or full transcript to video can result in a keyword rich page for site crawlers. In addition to the SEO benefits, there are certainly usability benefits for site visitors that have any visual or hearing impairments. There are a few ways to accomplish this:
Transcript in the page’s content
Place the transcript in normal HTML formatting somewhere near the video. This is the lowest-effort approach, however, the more text on the page, the more likely Google is to classify the page as “text with some video” rather than “video”. More advanced web developers can set up scripts to coordinate with the video’s timing to display blocks of transcript as they are being repeated in the actual video - - just remember to make sure the transcript text is in the same source code as the video itself.
Transcript on a separate page
Rather than have the transcript on the same page as the video, create a link to another page – this has the advantage of creating some cross-linking, which Google likes. The major drawback here is that the user has to focus on a separate page to see the transcript, rather than the page with the video.
Use structured data
As previously written in this space, structured data is content-specific detail that can help improve search engines' understanding of that content. One of the tags in the video schema is for transcript - http://schema.org/transcript. Note that this schema format is tag content, so the actual transcript text will not appear visible to a site visitor.