css slide menu

ul {
  list-style: none;
  position: relative;
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

ul li {
  position: relative;
  float: left;
  padding: 20px;
  margin-right: 5px;
  border: 1px solid #eee;
  box-sizing: border-box;
  cursor: pointer;
}
ul li ul {
  position: absolute;
  overflow: hidden;
  left: 0;
  top: 100%;
  transition: 250ms;
  max-height: 0;
  background: #000;
  color: #fff;
}

ul li:hover ul {
  max-height: 300px
}