It's an HLS video, which comes in the form of a text manifest file (.m3u8). The manifest file can define subtitle segments (.vtt, 608, 708) in addition to video segments (.ts) - it's the job of the video player to stitch these together to play at the same time. Safari & Edge have their own HLS implementations but every other browser uses a third party library. Server-side transcoding is responsible for making these manifest files from video & subtitle files.

Also, this video is in Flash