javascript getElementsByClassName() always returns none?

You might need to loop through the results, like this:

var divs = document.getElementsByClassName('source');
for(var i=0; i<divs.length; i++) { 
  divs[i].style.display='block'
}

And also as @ionoy mentioned, use display attribute. I hope that helps.

http://jsfiddle.net/erick/rb7bn/1/