CSS : Uncaught TypeError: Cannot set property 'className' of null

CSS : Uncaught TypeError: Cannot set property 'className' of null
To Access My Live Chat Page,
On Google, Search for "hows tech developer connect"

I have a hidden feature that I promised to tell you about.
This is a YouTube's feature which works on Desktop.
First, Make sure the video is currently in playing mode.
After that, type the word 'awesome' on your keyboard.
It will change your youtube progress bar into a flashing rainbow.

Here's a short introduction about myself,
Good day, I am Delphi.
I am here to help you get the answers you are seeking.
CSS : Uncaught TypeError: Cannot set property 'className' of null
If you have specific questions that need answers, please don't hesitate to comment or chat with me.
If you have an answer or any relevant information to share, please comment below.
Your answer will be acknowledged and appreciated, and I will 'heart' it as a sign of gratitude.
Cannot 'className' of : Uncaught property CSS null set TypeError: