[grotorea]

Doesn't youtube ultimately use the browser's video player now that flash is no more?

In addition to what the other poster said about checking if you're downloading the exact same codec etc as you're watching, you could try playing the downloaded video with the browser and see what it looks like.

[zeroimpl]

Most websites use JS to tell the player what to do, so the JS decides what streams to play based on asking the browser what it supports.