Incremental subtitle display support (Karoake) with webvtt

[clort81]

Expected behavior of the wanted feature

The webvtt format appears to support the kind of animated highlighting of subtitles that is known from the 'karoake player' space. It'd be interesting to add this feature to mpv.
Here's a sample .webvtt with incremental subs:
`
WEBVTT

1
00:00:00.000 --> 00:00:07.000
This <00:00:01.000>text <00:00:02.000>will <00:00:03.000>appear <00:00:04.000>over <00:00:05.000>6 <00:00:06.000>seconds.

2
00:00:07.000 --> 00:00:09.500
Hello<00:00:07.500> world<00:00:08.200>!

3
00:00:09.500 --> 00:00:12.000
This<00:00:09.500> is<00:00:10.100> a<00:00:11.200> test<00:00:11.900>!

`

Alternative behavior of the wanted feature

No response

Log File

No response

Sample Files

No response

[llyyr]

This is a feature request for FFmpeg's webvtt parser/ASS writer, not much mpv can do here. mpv doesn't have a native webvtt renderer (as that would require implementing a web engine in mpv), it just renders webvtt converted to ass via FFmpeg.