video element should clarify that captions must remain visible

[LJWatson]

Migrated from Bugzilla 13437
https://www.w3.org/Bugs/Public/show_bug.cgi?id=13437

current text:

"User agents may allow users to view the video content in manners more suitable to the user (e.g. full-screen or in an independent resizable window). As
for the other user interface features, controls to enable this should not interfere with the page's normal rendering unless the user agent is exposing
a user interface. In such an independent context, however, user agents may make full user interfaces visible, with, e.g., play, pause, seeking, and volume
controls, even if the controls attribute is absent."

recommended change:

"User agents may allow users to view the video content in manners more suitable to the user (e.g. full-screen or in an independent resizable window).
Captions, subtitles or other additional visual tracks must remain available and visible at all times. As for the other user interface features, controls
to enable this should not interfere with the page's normal rendering unless the user agent is exposing a user interface. In such an independent context,
however, user agents may make full user interfaces visible, with, e.g., play, pause, seeking, and volume controls, even if the controls attribute is absent."

[chaals]

This is non-editorial as it adds a requirement.

Also, I think the additional requirement

Captions, subtitles or other additional visual tracks must remain available and visible at all times

should be a should not a **must. Not least, because it is possible that they don't fit. That might be what "available" is trying to address, but it seems that this proposal should be cleaned up a bit.