How can I make a banner that can be edited and updated in real-time?

I want to have a banner with text. I want to be able to edit the text on the banner. When I edit the text, I want the new text to display instantaneously, for everyone who is viewing the banner. They don’t need to refresh or anything; the banner changes as soon as I edit it.

This banner will also need a password so that I am the only one capable of editing it.

Is this possible? Where would I begin to accomplish this? If it’s a simple, five-minute job, could someone please create it for me?