webVTT line: and vertical position

[btsimonh]

WebVTT usage is severely hobbled by the inconsistant implementation of line: interpetation.

e.g.

00:00:00.000 --> 00:00:10.000 line:50%
Hello world.
1
2
3

According to the spec, the top of Hello should be at the mid vertical of the video, because lineAlign defaults to start.
Chrome/Chromecast/Opera do something completely different. They place the text such that 0% represents agains the top, and 100% represents text against the bottom (logical, but not to spec).
Firefox and Exoplayer (see google/ExoPlayer#6682) apparently DO default to lineAlign=start.

If you replace line:50% with line:50%,start, Chrome,Opera fail to parse, and position defaults. Firefox parses, but interprets center as end, and end as center.

Q:
1/ Does anyone know of a proposed change to behave like Chrome/Opera? If so, please reference.
2/ Does anyone have suggestions about how to achieve consistent vertical positioning across browsers?