<nav class="menu">
  <a href="#" class="menu__item" data-tooltip="Dashboard">
    <i class="material-icons">home</i>
  </a>
  <a href="#" class="menu__item menu__item--active" data-tooltip="Music Library">
    <i class="material-icons">music_note</i>
  </a>
  <a href="#" class="menu__item" data-tooltip="Statistics">
    <i class="material-icons">timeline</i>
  </a>
  <a href="#" class="menu__item" data-tooltip="Account Security">
    <i class="material-icons">https</i>
  </a>
  <a href="#" class="menu__item" data-tooltip="Settings">
    <i class="material-icons">settings</i>
  </a>
</nav>
<h1>contents here</h1>

html {
  /* set space for menu to appear, or everything in html will covered 75px by menu  */
  margin-left: 75px;
}

.menu {
  position: fixed;
  top: 0;
  left: 0;
  width: 75px;
  height: 100%;
  background: #eeeeee;
  font-size: 16px;
  font-family: sans-ferif;
}

.menu__item {
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 15px 0;
  text-decoration: none;
  color: #666666;
  transition: background 0.3s;
}

.menu__item > i {
  font-size: 2.2em; /* 2.2*16px */
}

.menu__item--active {
  color: #000000;
  background: #dddddd;
}

.menu__item:hover {
  background: #dddddd;
}

/* create tooltip after each menu item  */
.menu__item::after {
  position: absolute;
  display: inline-block; /* ::before, ::after are inline as default
  
  /*  makes tooltip vertically center within each one of menu items  */
  top: 50%;
  transform: translateY(-50%);
  
  /* starting tooltip 100% left of menu item (just the right of them)   */
  left: 100%;
  
  /* linked to the HTML attribute value  */
  /* 使用 ::before 和 ::after 一定要有 content 否則不會發生作用。 */
  content: attr(data-tooltip);
  
  /* set margin between menu item and tooltip  */
  margin-left: 15px;
  
  /* makes words in same line   */
  white-space: nowrap;
  
  padding: 7px 12px;
  
  /* 90% of current font size   */
  font-size: 0.9em;
  font-weight: bold;
  background: rgba(0, 0, 0, 0.8);
  border-radius: 15px;
  color: #fff;
  
  /*  invisible (can't see it but can click on it)  */
  opacity: 0;
  /* can't click  */
  visibility: hidden;
  transition: opacity 1s;
}

/* show tooltip if hover on menu item */
.menu__item:hover::after {
  opacity: 1;
  visibility: visible;
}