Test if animated 3-line captions are easier to read

[bert-github]

Instead of showing one line of subtitles at a time, we could show two with an animation, like you sometimes see on TV when a live event is captioned: Two lines of captions are visible, the third line is added at the bottom and then in the time of half a second the three lines move up until the first line has disappeared.

It's not so easy to handle all cases (lines that are too long, recognizing pauses…), but a reasonable approximation can be done by removing the first line if there are three and setting a CSS animation on what's now the first line, before adding the new line.

My own impression on a brief test is that it is more distracting than helpful, but it is worth testing with other people.