onload=function() {
  setTimeout("blink_text('blink_text')",500);
}

function blink_text(div_id) {
  // If the text is hidden then display it
  if (document.getElementById(div_id).style.visibility=="hidden") {
    document.getElementById(div_id).style.visibility="visible"
  // else hide the text
  } else {
    document.getElementById(div_id).style.visibility="hidden"
  }
  // use the timeout to run the code every 500 miliseconds
  setTimeout("blink_text('"+div_id+"')",500);
  
}