The Web Content Accessibility Guidelines version 2.2 require four pieces for a video to be considered accessible. In brief, those guidelines are:

The Tags Panel displays the primary structure of the document. While the Order Panel provides tools to restructure what content is grouped together, how it is labeled, and when it is read, the Tags Panel gives more granular control over the latter two aspects. The order of the tags is the reading order of the document, moreso than that displayed by the Order Panel, and each tag serves a specific semantic purpose (besides grouping tags like <Part> and <Sect>).

The PDF is a curious case for accessibility. On one hand, it has the potential to be a useful standard for both accessible reading and for printing. On the other, it is rarely utilized in an accessible manner and has many obscure issues and odd behaviors that can lead to problems.