You can use these shortcuts on Windows, Mac, and Linux, across Chrome, Firefox, and Microsoft Edge. They also work on YouTube videos embedded on other websites, but their behavior might sometimes be erratic.
• J Key
The J key take video backward by 10 seconds
• L Key
The L key takes the video forward by 10 seconds.
• K Key
The K key allows you to play/pause YouTube videos, just like spacebar does. Sandwiched between the J and L keys, it functions like the Play/Pause button found in media player controls on various keyboards.
• 1-9 Keys
The 1-9 keys allow you to skip to certain percentages in the video, so pressing 1 takes you 10% into the video, pressing 2 takes you 20% into the video, etc.
• O Key
The 0 key takes you to the start of the video. This works both when you’re playing the video and when you have paused it.