[ruslan_mart]

HTML:

<input type="button" value="+1" onclick="addNum()" />
<span id="add_num">0</span>

javascript:

var elem = document.getElementById('add_num'), num = +elem.innerHTML;

function addNum()
{
   elem.innerHTML = num++;
   if(num == 10)
   {
      alert('Вы досчитали до 10!');
      num = -1;
   }
}