javascript setAttribute

There are two ways to do this (correctly). You can use this cross-browser method here:

elem_pop.appendChild( document.createTextNode('coming') );

The other way is to use elem_pop.textContent and/or elem_pop.innerText, but these unfortunately don't work cross-browser. IE only supports innerText, Firefox, Chrome only support textContent - Opera is the only one that supports both.

You could use elem_pop.innerHTML, HOWEVER be aware that innerHTML inserts HTML markup, NOT just text into the page. This means that it won't escape any special characters into entities. It also replaces any child nodes automatically and can destroy event listeners.

I'd recommend the first (appendChild()) method, but if you want to use textContent/innerText you could place this at the top of your script:

HTMLElement.prototype.__defineGetter__('innerText', function(){ return this.textContent; });
HTMLElement.prototype.__defineSetter__('innerText', function(t){ this.textContent=t; });

That will give Firefox innerText support and you can use it all you like.

The span element does not have a value attribute. Try setting its innerHTML instead:

elem_pop.innerHTML = "coming";

Use .innerHTML instead here, like this:

function modifyDOM(){
  var elem_pop = document.getElementById("pop_title");
  elem_pop.innerHTML = "coming";
}

You can try it here. Setting the value attribute would be for something like an <input> element, when you want to replace the html within an element's tags, use .innerHTML instead.