HTML : Why 'Prevent this page from creating additional dialogs' appears in the alert box?

HTML : Why "Prevent this page from creating additional dialogs" appears in the alert box?
To Access My Live Chat Page,
On Google, Search for "hows tech developer connect"

As promised, I'm going to share a hidden feature with you.
This is a YouTube's feature which works on Desktop.
First, Make sure this video is playing.
Then, type the letters 'awesome' on the keyboard.
You will see a flashing rainbow instead of a regular progress bar on YouTube.

Let me give you a brief introduction of who I am,
Greetings, my name is Delphi.
I help you answer your questions.
HTML : Why "Prevent this page from creating additional dialogs" appears in the alert box?
If you have specific questions that need answers, please don't hesitate to comment or chat with me.
If you have additional information or an answer to share, please comment below.
Your answer will be recognized and valued, and I will 'heart' it to show my appreciation.
this "Prevent the additional : HTML dialogs" from appears Why creating page box? alert in