Overlapping menu on smartphone

Overlapping menu on smartphone

HTML

<html>
  <head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
    <link rel="stylesheet" href="style.css">
  </head>
  <body>
    <div id="container">
      <div id="header">
        <div id="menu">
          <div class="nav">
            <a class="active" href="index.php">HOME</a>
            <a href="index.php">BIO</a>
            <a href="index.php">MEDIA</a>
            <a href="index.php">CONTACT</a>
          </div> <!-- end nav -->
        </div> <!-- end menu -->
      </div> <!-- end header -->
      <div id="content">
        <p>
          Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut 
          laoreet dolore magna aliquam erat volutpat.
          Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea 
          commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, 
          vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit 
          praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi. Nam liber tempor cum soluta 
          nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum.
          Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim 
          placerat facer possim assum.
        </p>
      </div> <!-- end content -->
    </div> <!-- end container -->
  </body>
</html>

CSS

body {
  background-color: #111111;
  color: #eeeeee;
}

#container {
  height: 100%;
}

#header {
  position: center;
  width:100%;
  background-image: url("img/header_bg_merged_smaller.jpg");
  background-repeat: no-repeat;
  background-position: top center;
  background-size: cover;
  height: 0;
  padding-top: 60.53%;
}

.nav {
  position: center;
  text-align: center;
  font-size: 1.1em;
  display: block;
  font-weight: bold;
  color: #eeeeee;
  width:100%;
}

.nav a:link {
  background-color: #444444;
  color: #eeeeee;
  padding: 15px;
  height: 100%;
  display: inline-block;
  margin-bottom: 15px;
}

.nav a:hover {
  background-color: #999999;
  color: #ff0000;
}

.nav a:visited {
  background-color: #444444;
  color: #eeeeee;
}

.nav a:active {
  background-color: #555555;
  color: #eeeeee;
}

@media (min-aspect-ratio: 4/3) {
  #content {
    padding-left: 2em;
    padding-right: 2em;
    padding-top: 5%;
    font-size: 1.1em;
    width: 60%;
  }
}

@media (max-aspect-ratio: 3/4) {
  #content {
    padding-left: 2em;
    padding-right: 2em;
    padding-top: 5%;
    font-size: 1.1em;
    width: 100%;
  }
}