How can HTML5 video's byte-range requests (pseudo-streaming) work?

How can HTML5 video's byte-range requests (pseudo-streaming) work?

pIf you play an HTML5 video for a video that is hosted on a server that
accepts range requests, then when you try to seek ahead to a non-buffered
part of the video you'll notice from the network traffic that the browser
makes a byte range-request. I'm assuming that the browser computes the
byte by knowing the total video size ahead of time and assuming a constant
bitrate (if you click half-way in the progress bar, then it will request
the byte at the half-way point). But especially if the video is variable
bitrate, it seems unlikely that the byte it requests could really
correspond to the time-point that the user clicked on, and the byte would
likely fall in the middle of a frame./p pHow does the browser know what
the beginning of the next frame is, once it's begun fetching at some
arbitrary byte?/p