Question on printing remote text onto a document with JS

[TheAngryTomato]

One of my clients I am working for would like a little 'deals' banner on their website. They would like it on every page, and to be able to change it. I figured the best way to do that is have it link to JS file, where the ext will be stored and then printed when the file is opened by the web browser. I am new to JS, so this is what I've come up with:

<script language="JavaScript" src="deals.js"></script>

This is the file deals.js:

var txt = "Text goes here"
document.write(txt)

Also, is there a way I can put HTML code into the txt variable so I can have links/bolding/coloring?

Any help would be great.

-TheAngryTomat