nav.css

* {
  box-sizing: border-box;
}

#sizeTest {
  -webkit-transition: width .001s;
     -moz-transition: width .001s;
      -ms-transition: width .001s;
       -o-transition: width .001s;
          transition: width .001s;
  width: 0;
}

#sizeTest:after {
  content: "small";
  display: none;
}

body {
  margin: 0;
}

nav button {
  float: left;
  left: 5px;
  position: absolute;
  top: 5px;
}

nav a {
  color: #fff;
  font-weight: bold;
  text-decoration: none;
}

nav ul {
  background: #333;
  display: none;
  list-style: none;
  margin: 0;
  padding: 10px 0;
  width: 100%;
}
nav li {
  padding: 10px 0;
  text-align: center;
  width: 100%;
}
nav ul ul {
  background: #555;
  display: none;
  width: auto;
}
nav li li {
  float: none;
}

@media all and (min-width:500px) {
  #sizeTest {
    width: 500px;
  }
  #sizeTest:after {
    content: "large";
  }
    
  nav button {
    display: none;
  }
  nav ul {
    display: block;
    float: left;
  }
  nav li {
    float: left;
    position: relative;
    text-align: left;
    width: auto;
  }
  nav li:hover ul {
   display: block;
  }
  nav li a {
    padding: 0 30px;
    white-space: nowrap;
  }  
  nav ul ul {
    background: #333;
    position: absolute;
  }
}