list.css

*{
  margin: 0;
  padding: 0;
  font-family: sans-serif;
  list-style: none;
  text-decoration: none;
}

.middle{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}
.menu{
  width: 300px;
  border-radius: 8px;
  overflow: hidden;
}
.item{
  border-top: 1px solid #2980b9;
  overflow: hidden;
}
.btn{
  display: block;
  padding: 16px 20px;
  background: #3498db;
  color: white;
  position: relative;
}
.btn:before{
  content: "";
  position: absolute;
  width: 14px;
  height: 14px;
  background:#3498db;
  left: 20px;
  bottom: -7px;
  transform: rotate(45deg);
}
.btn i{
  margin-right: 10px;
}
.smenu{
  background: #333;
  overflow: hidden;
  transition: max-height 0.3s;
  max-height: 0;
}
.smenu a{
  display: block;
  padding: 16px 26px;
  color: white;
  font-size: 14px;
  margin: 4px 0;
  position: relative;
}
.smenu a:before{
  content: "";
  position: absolute;
  width: 6px;
  height: 100%;
  background: #3498db;
  left: 0;
  top: 0;
  transition: 0.3s;
  opacity: 0;
}
.smenu a:hover:before{
  opacity: 1;
}
.item:target .smenu{
  max-height: 10em;
}