The only time I notice it not working that way still is if my internet cuts out or the page itself is having problems and it won't load at all. Otherwise it loads the entire video pretty quickly. Like don't even have to pause it to see the gray line getting lighter because it goes so fast it's done loading a 20 minute video within the first 30 seconds of said video.
Now, if I am watching cable TV on the other hand... That shit buffers like crazy. And it's even weirder that I have cable internet. How is my internet faster than the TV when they use the same lines?
It's called DASH protocol and it's designed to only have a small adaptive bandwidth buffer. The whole video will never buffer, only a small percentage of it.