The native <video> and <audio> tags buffer a few megabytes at a time. This is easy to see by opening a video file in the browser (not from file://) and watching the network tab.
Depends on how you set it up apparently, I assume it changed over time:
preload= "metadata" - is the default which is 2-3% of the video.
preload="none" - no video will be downloaded on page load
preload="auto" - the entire video is downloaded. (Some browsers do not do this, and only download partial videos anyway)