XML : Having a JavaScript file display certain text in my HTML File, but through xml?

So what I am trying to do is when I open a .xml file and edit said tags inside, the JavaScript file will display the text. What I'm asking is what is the best method to do this? Through JavaScript, or .xml?

My JavaScript file:

  var DailyMessage = 'Text here';    document.write(DailyMessage);  

My .xml code: I have absolutely no clue

HTML Snippet

  <script src="Blog.js"></script>  

The reason I'm asking this is because my friend is asking me to help him with his Blog site, but wants an easy way to edit the daily message without the hassle of having to scroll through the html.