The subtitles are shown with a delay with the HTML5 player.
Here's an example of a video with synchronized subtitles shown too late when played with the HTML5 player:
https://commons.wikimedia.org/wiki/File:Around_the_Corner_%281937%29_24fps_selection.webm
Some subtitles are shown at the right time, but most have a delay.
It seems at least partially caused by the subtitles fading animation.