If you watch a lot of YouTube on your Mac, it pays to learn some of the keyboard shortcuts to have better control over the videos you watch on YouTube.
Play / Pause. Press the Space or K key to toggle play and pause.
Skipping. Press the number keys 0 through 9 on your keyboard to move forward and back a percentage of the video duration. Press 0 to go to the start, 1 to go to 10%, 2 to go to 20% and so on.
Rewind & Forward. For more fine grain control, you can press J and L to move forward and back 10 seconds at a time.
Full Screen. Press F to enter full screen mode. Press Esc to exit.
Mute. Press M to mute the sound.