options for displaying notifications in the browser header?

I need a modern option for displaying a notification header to the user. The implementation needs to be cross-browser compatible and as lightweight as possible. My manager recommended the use of VanillaJs so a vanillaJs plugin would be ideal but if you can make a case for a better different solution then I can pass that along to my manager.

Also, the intent of this solution is to display the notification in a wide variety of websites across the enterprise. From a ux perspective do you think this design may be better implemented as a popup? I'm wondering if this may appear less shoehorned than displaying a modern notification header in an app with an older ui design.