var font_size = 12; 
var MAX = 90; 
var MIN = 12; 

function CambiarFuente(num) 
{ 
        font_size += num; 
if(font_size > MAX){ 
        font_size = MAX; 
} 
if(font_size < MIN){ 
        font_size = MIN; 
} 
  var TamFuente = document.getElementById("CambiarTamanyo") 
 TamFuente.style.fontSize = font_size + "px"; 
} 
CambiarFuente(0);