* {
  margin: 0;
  padding: 0;
  font-family: 'Microsoft YaHei Light UI', 'Microsoft YaHei Light', 'Microsoft YaHei', 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 {
  position: relative;
  display: block;
  padding: 16px 20px;
  background: #3498db;
  color: #fff;
}

.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 .3s;
  max-height: 0;
}

.smenu a {
  display: block;
  padding: 16px 26px;
  color: #fff;
  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: .3s;
  opacity: 0;
}

.smenu a:hover::before {
  opacity: 1;
}

.item:target .smenu {
  max-height: 14em;
}