The <audio> and <video> elements in SVG need to have a peer element to contain timed text to support captions, subtitles and the text of descriptions. While an approximation to captions can be done using <animation>, this is not a good substitute to using a dedicated timed text format and there are no controls for the user to find them.
Allow <foreignObject> to have the Attributes for runtime synchronization so that for example an external TTML file may be referenced, or an embedded TTML resource.
Add an explicit <timedText> element equivalent to <video> to provide such references.
Add system properties for captions, descriptions and subtitles in order to control the rendering of these formats.