[devote]

Не понимаю, зачем столько JavaScript кода.

<style>
  #bar {
    position: absolute;
    top: 0;
    left: 0;
    height: 3px;
    background: red;
    box-sizing: border-box;
    -webkit-box-shadow: 1px 1px 6px 0px #333;
    -moz-box-shadow: 1px 1px 6px 0px #333;
    box-shadow: 1px 1px 6px 0px #333;
    -o-transition: 1s width ease;
    -ms-transition: 1s width ease;
    -moz-transition: 1s width ease;
    -webkit-transition: 1s width ease;
    transition: 1s width ease;
    width: 0;
  }
</style>
<div id="bar" class="zero-width"></div>
<button onclick="document.getElementById('bar').style.width='30%';">(30)</button>
<button onclick="document.getElementById('bar').style.width='60%';">(60)</button>
<button onclick="document.getElementById('bar').style.width='100%';">(100)</button>