[ruslan_mart]

1.

document.getElementById('mySpan').innerHTML += 'This is text!';

2.

document.getElementById('mySpan').appendChild( document.createTextNode('This is text!') );

3.

document.getElementById('mySpan').appendChild(function() {
   var a = document.createElement('a');
   a.href = 'http://javascript.ru';
   a.innerHTML = 'This is new link!';
   a.target = '_blank';
   return a;
}());