In my opinion, when tested in the browser banner is not shown.
And gives the message "ErrorMessage banner ads not supported in web platform".
But it should work in real application.
I am warning you!
You should not use AdMob so frivolously. Always check before creating a banner, maybe it's already loaded.
And one more tip! Forget about such advertising as a banner. This is very bad advertising and it makes very little money. VERY LITTLE!
And the banner spoils retention in the game.