In JavaScript there’s a .join() method on arrays to get a string, which you can provide the delimiter to. In your case it’d look like this:
var myArray = ['css','html','xhtml','html5','css3','javascript','jquery','lesscss','arrays','wordpress','facebook','fbml','table','.htaccess','php','c','.net','c#','java'];
var myString = myArray.join(', ');
You can test it out here