[Vlasenko Fedor]

<style>
  #sq {
    background-color:red;
    width: 50px;
    height: 50px;
  }
  input {
    position: fixed;
    left: 100px;
    top: 20px;
    width: 50px;
  }
</style>
<div id="sq"></div>
<input type="button" value="Down" onclick="go(this)" />
<script>
  var go = function (el) {
      var divsq = document.getElementById("sq"),
        mtop = parseInt(divsq.style.marginTop) || 0,
        state = el.value == "Up",
        tId = setInterval(function () {
          mtop += state ? -5 : 5;
          divsq.style.marginTop = mtop + "px";
          if (!mtop || mtop == 80) {
            el.value = state ? "Down" : "Up";
            clearInterval(tId);
          }
        }, 30);

    };
</script>