<meta name="viewport" content="width=device-width, initial-scale=1">

.navbar {
    overflow: hidden;
    background-color:rgba(0,0,0,0.50);
    font-family: Arial, Helvetica, sans-serif;
}

.navbar a {
    float: left;
    font-size: 16px;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

.dropdown {
    float: left;
    overflow: hidden;
}

.dropdown .dropbtn {
  position: relative;
    font-size: 16px;    
    border: none;
    outline: none;
    color: white;
    padding: 14px 16px;
    background-color: inherit;
    font-family: inherit;
    margin: 0;
}

.navbar a:hover, .dropdown:hover .dropbtn {
    background-color: red;
}

.dropdown-content {
    display: none;
    position: absolute;
    top: 50px;
    background-color: #f9f9f9;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
    z-index: 1;
}

.dropdown-content a {
    float: none;
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
    text-align: left;
}

.dropdown-content a:hover {
    background-color: #ddd;
}

.dropdown:hover .dropdown-content {
    display: block;
}
/*手機版上以以ICON界面呈現*/
@media only screen and (max-width:640px) {
.dropbtn{
  display:inline-block;
  background:url(icon_menu_48.png) no-repeat;
  width: 48px;
  height: 48px;
  overflow: hidden;
  text-indent: -150px;
  }
}

<div class="navbar">
    <div class="dropdown">
    <button class="dropbtn">MENU
       <i class="fa fa-caret-down"></i>
    </button>
    <div class="dropdown-content">
      <a href="#">Link 1</a>
      <a href="#">Link 2</a>
      <a href="#">Link 3</a>
    </div>
  </div>
</div>