How do I make the sound from youtube page play on the speaker which has already bluetooth paired?

I have bluetooth paired my speaker, but safari won't play the sound to it. Safari (Chrome too) still uses my macbook to play the sound. But if I use a headphone, Safari can play the sound to the headphone. How do I let them play the sound from my speaker. Safari can only play the sound to the headphone not speaker. Both iTune and VLC have an option to choose audio output device, but browsers do not have this option. The Chrome help document says I need to "**Cast to a device**", but unfortunately I didn't see that option on mac (I have tried two macbooks)