function shou ()
  {
    document.getElementsByTagName('div').style.display = "block";
  }

<button onclick="shou()"> Показать</button>

function show() {
  var elements = document.getElementsByTagName("DIV");
  
  for(var i = 0, leng = elements.length; i < leng; i++) {
     elements[i].style.display = "block";
  }
}

<div style='display: none; width: 100px; height: 100px;  background:red'></div>
<button onclick='show()'> Показать</button>

<script>
function show() {
  document.getElementsByTagName('div')[0].style.display = "block";
  return false;
}
</script>