I don't need audio, but mp4 file animations tend to have better quality and smaller filesize.
It's a mystery I'll never understand, but for some reason the internet seems to always converge on using the least efficient means possible of transmitting information.
Images of just words, when plain (or rich) text would be better. Resulting in lower quality and much larger filesize.
GIFs instead of proper video files. Resulting in lower quality and much larger filesize.
What's one of the best places to find and hear nearly any song in the world for free? It's at YouTube in video format. Resulting in a much larger filesize than necessary.