Indicate the viewport's position relative to rendered content (e.g., the proportion of an audio or video clip that has been played, or the proportion of a Web page that has been viewed).
Sufficient techniques
The user agent may calculate the relative position according to content focus position, selection position, or viewport position, depending on how the user has been browsing.
The user agent may indicate the proportion of content viewed in a number of ways, including as a percentage or as a relative size in bytes. See checkpoint 1.3 for more information about text versions of messages to the user, including messages about position information.
Normative inclusions and exclusions
For two-dimensional spatial renderings, relative position includes both vertical and horizontal positions.
This checkpoint does not require the user agent to present information about retrieval progress. However, for streaming content, viewport position may be closely tied to retrieval progress.