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HTML 

              
                <aside id="archives" class="widget widget_archive">
  <h3 class="widget-title">Archivos</h3>
  <ul>
    <li><a href="#">2015</a>
      <i class="fa fa-angle-down"></i>
      <ul>
        <li><a href="#">febrero 2015</a></li>
        <li><a href="#">enero 2015</a></li>
      </ul>
    </li>
    <li><a href="#">2014</a>
      <i class="fa fa-angle-down"></i>
      <ul class="hide">
        <li><a href="#">diciembre 2014</a></li>
        <li><a href="#">noviembre 2014</a></li>
        <li><a href="#">marzo 2014</a></li>
        <li><a href="#">febrero 2014</a></li>
        <li><a href="#">enero 2014</a></li>
      </ul>
    </li>
    <li><a href="#">2013</a>
      <i class="fa fa-angle-down"></i>
      <ul class="hide">
        <li><a href="#">diciembre 2013</a></li>
        <li><a href="#">septiembre 2013</a></li>
      </ul>
    </li>
  </ul>
</aside>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Lato);

body {
  font-family: 'Lato', sans-serif;
}

.fa-angle-down {
  transition: all .3s ease-in-out;
}

.widget {
  width:60%;
  margin:50px auto 0 auto;
  border:4px solid rgba(10,10,10,0.05);
  padding:10px;
  border-radius:10px;
  -webkit-box-shadow: 10px 10px 45px -20px rgba(0,0,0,0.75);
  -moz-box-shadow: 10px 10px 45px -20px rgba(0,0,0,0.75);
  box-shadow: 10px 10px 45px -20px rgba(0,0,0,0.75);
}

.widget-title {
  font-weight:bold;
  text-transform:uppercase;
  color:#636363;
  padding-bottom:10px
}

.widget a {
  color:#9b9b9b;
  text-decoration:none;
  padding:10px 0 10px 10px;
  display:inline-block;
}

.widget i {
  color: #9b9b9b;
  padding:10px 16px;
  float:right;
  line-height:1.5;
}

.widget i:hover {
  cursor: pointer;
  color:#444;   
}

.widget a:hover {
  color:#444
}

.widget ul {
  list-style-type:none;
  margin:0;
  padding:0;
  background-color:#eee;
}

.widget ul li:first-child .fa-angle-down {
  -moz-transform: rotate(180deg);
  -ms-transform: rotate(180deg);
  -webkit-transform: rotate(180deg); 
  transform: rotate(180deg);
}

.widget ul {
  line-height:1.5;
}

.widget li {
  border-top:1px solid #ccc;
}

.widget li li {
  line-height:1.5;
  border-top:1px solid #eaeaea;
  background-color:#fff;
}

.widget > ul li:first-child 
li a:first-child {
  display:inline-block;
}

.hide {
  display:none;
}
!

JS 

              
                /* 

Reviewed on http://codereview.stackexchange.com/questions/83940/minimalist-accordion-implementation-using-vanilla-javascript 

http://sonnyt.com/javascript-check-if-element-has-class/

*/

var el = document.getElementById("archives");
var hook = el.getElementsByClassName("fa");
var rotated;

for (var i = 0; i < hook.length; i++) {
	hook[i].addEventListener("click",Toogle);
}

function Toogle() {
    'use strict';
  if (this.nextElementSibling.nodeType === 1 && hasClass(this.nextElementSibling,"hide")) { 
    if (this.nextElementSibling.classList) {
      this.nextElementSibling.classList.remove('hide');
    } else {
      this.nextElementSibling.className = this.nextElementSibling.className.replace(new RegExp('(^|\\b)' + "hide" + '(\\b|$)', 'gi'), ' ');
    }
    rotated = false;
  }
  else {    
    this.nextElementSibling.className += " " + "hide";
    rotated = true;
  }
  
  var deg = rotated ? 0 : 180;
    
  this.style.transform = 'rotate('+deg+'deg)';     
}

function hasClass (el, className){
  return el.className && new RegExp("(^|\\s)" + className + "(\\s|$)").test(el.className);
}
!
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