Why some video websites don't use gpu?

when i use youtube it uses my cpu but i can switch it to html 5 and i get the gpu to work.

 

so why is it some sites don't use the gpu to play video?

because the vast majority of computers do not have a dedicated graphics card, and websites cater to the majority, not the minority. and PC gamers with nice graphics cards are the minority

They need to be multi CPU and GPU.... I'm running on a laptop right now i5-2430M with 81 Tabs, Virtual machine with 25 Applications open and over 30 tabs and I'm running about 15 applications on my physical machine....