<nav>
  <ul class="tabs">
    <li class="active"><a href="#home">Home</a></li>
    <li><a href="#about">About</a></li>
    <li><a href="#contact">Contact</a></li>    
    <li><a href="#">Really long string sample</a></li>
  </ul>
</nav>

body {
  font-family: Arial, Verdana, Helvetica, sans-serif;
  font-size: 1em;
  overflow: hidden;
  min-height: 325px;
}
nav {
  width: 800px;
  margin: 0 auto;
}

/*Menu*/
ul.tabs {
  float: left;
  clear: both;
  width: 800px;
  list-style: none;
  border-bottom: 2px solid #00aeef;
  position: relative;
  font-size: 1.061em;
  padding: 0;
}
ul.tabs li {
  float: left;
}
ul.tabs li a {
  display: block;
  text-align: center;
  text-decoration: none;
  color: #666;
  padding: 12px 20px;
  min-width: 85px;
  font-weight: normal;
  transition: color 0.3s;
}
ul.tabs li.active a {
  color: #00aeef;
}

#magic-line {
  position: absolute;
  bottom: -2px;
  left: 0;
  width: 125px;
  height: 4px;
  background: #00aeef;
  transition: all 0.3s;
}
/*Clear Fix*/
.tabs:after {
  visibility: hidden;
  display: block;
  content: "";
  clear: both;
  height: 0;
}
*:first-child + html .tabs {
  zoom: 1;
}

$(function() {
  'use strict';
  /*Activate default tab contents*/
  var leftPos, newWidth, $magicLine;

  $('.tabs').append("<li id='magic-line'></li>");
  $magicLine = $('#magic-line');
  $magicLine.width($('.active').width())
    .css('left', $('.active a').position().left)
    .data('origLeft', $magicLine.position().left)
    .data('origWidth', $magicLine.width());

  $('.tabs li a').click(function() {
    var $this = $(this);
    $this.parent().addClass('active').siblings().removeClass('active');
    $magicLine
      .data('origLeft', $this.position().left)
      .data('origWidth', $this.parent().width());
    return false;
  });

  /*Magicline hover animation*/
  $('.tabs li').find('a').hover(function() {
    var $thisBar = $(this);
    leftPos = $thisBar.position().left;
    newWidth = $thisBar.parent().width();
    $magicLine.css({
      "left": leftPos,
      "width": newWidth
    });
  }, function() {
    $magicLine.css({
      "left": $magicLine.data('origLeft'),
      "width": $magicLine.data('origWidth')
    });
  });
});