为什么我的 div 元素重叠？

/* Mobile Navigation */

#mobile-nav {
  display: none;
  height: 75px;
  background-color: #262626;
}

#mobile-nav .mobile-nav-tgl-btn {
  height: 50px;
  width: 50px;
  background-color: transparent;
  border: none;
  margin-left: 15px;
  margin-top: 12.5px;
  cursor: pointer;
}

#mobile-nav .mobile-nav-tgl-btn .bar {
  height: 7.5px;
  width: 45px;
  background-color: white;
  margin-top: 6.25px;
  margin-bottom: 6.25px;
  margin-left: -5px;
}

#mobile-nav .nav-links {
  text-align: center;
}

#mobile-nav .nav-links ul {
  padding-left: 0;
  margin-top: 4;
}

#mobile-nav .nav-links ul li {
  list-style-type: none;
  border: 2px solid black;
}

#mobile-nav .nav-links ul li a {
  text-decoration: none;
  color: black;
  font-size: 1.6em;
}


/* Nomral Navigation */

#norm-navbar {
  height: 75px;
  background-color: #262626;
}

#norm-navbar .norm-nav-links {
  padding-top: 16.5px;
  float: left;
}

#norm-navbar .norm-social-links {
  padding-top: 16.5px;
  float: right;
}

#norm-navbar a {
  font-size: 1.85em;
  /* 29.6px font-size */
  padding-left: 15px;
  padding-right: 15px;
  text-decoration: none;
  color: white;
}


/* Site header */

#header-section {
  height: auto;
  background-color: orange;
  padding: 0;
  margin: 0;
  width: 100%;
}

#header-section .header-elements {
  width: 75%;
  margin: 0 auto;
  text-align: center;
  padding-top: 100px;
  padding-bottom: 120px;
}

#header-section .header-elements img {
  height: 150px;
  width: 150px;
  padding-bottom: 30px;
}

#header-section .header-elements h1 {
  font-size: 3em;
}

#header-section .page-down-btn {
  width: 20%;
  margin: 0 auto;
  text-align: center;
  padding-bottom: 10px;
}

#header-section .page-down-btn p {
  margin-top: 0;
  margin-bottom: 5px;
}

#header-section .page-down-btn i {
  font-size: 1.5em;
}

#header-section .page-down-btn a {
  text-decoration: none;
  color: black;
}


/* About me section */

#about-me {
  width: 100%;
  position: absolute;
  height: auto;
}

#about-me .left-panel {
  width: 60%;
  text-align: center;
  float: left;
  height: 450px;
}

#about-me .left-panel h2 {
  font-size: 2.5em;
  margin-top: 10px;
  margin-bottom: 15px;
}

#about-me .left-panel .about-me-para {
  width: 75%;
  margin: 0 auto;
}

#about-me .left-panel .about-me-para p {
  margin: 0 auto;
  text-align: left;
  font-size: 1.5em;
}

#about-me .right-panel {
  width: 40%;
  float: right;
  background-image: url("http://cdn.zmescience.com/wp-content/uploads/2014/07/over_the_edge.jpg");
  height: 497px;
}


/* Portfolio/work section */

#work-section {
  height: 500px;
  width: 100%;
  background-color: orange;
  display: block;
}

<div id="mobile-nav">
  <button class="mobile-nav-tgl-btn">
        <div class="bar"></div>
        <div class="bar"></div>
        <div class="bar"></div>
      </button>
  <div class="nav-links">
    <ul>
      <li><a href="#">About</a></li>
      <li><a href="#">Work</a></li>
      <li><a href="#">Contact</a></li>
    </ul>
  </div>
</div>

<div id="norm-navbar">
  <div class="norm-nav-links">
    <a href="#">About</a>
    <a href="#">Work</a>
    <a href="#">Contact</a>
  </div>
  <div class="norm-social-links">
    <a href="#"><i class="fa fa-twitter" aria-hidden="true"></i></a>
    <a href="#"><i class="fa fa-instagram" aria-hidden="true"></i></a>
    <a href="#"><i class="fa fa-github" aria-hidden="true"></i></a>
    <a href="#"><i class="fa fa-codepen" aria-hidden="true"></i></a>
  </div>
</div>

<div id="header-section">
  <div class="header-elements">
    <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Google_%22G%22_Logo.svg/2000px-Google_%22G%22_Logo.svg.png" alt="Logo">
    <h1>Hi, I'm a website developer that loves to make <strong>stunning</strong> and <strong>functional</strong> websites.</h1>
  </div>
  <div class="page-down-btn">
    <p>Click to learn more</p>
    <a href="#"><i class="fa fa-arrow-down" aria-hidden="true"></i></a>
  </div>
</div>

<div id="about-me">
  <div class="left-panel">
    <div class="left-panel-cont">
      <h2>Lorem ipsum</h2>
      <div class="sm-sep"></div>
      <div class="about-me-para">
        <p class="para-1">
          Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor
          in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
        </p>

        <br>

        <p class="para-2">
          Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor
          in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
        </p>
      </div>
    </div>
  </div>

  <div class="right-panel">
  </div>
</div>

<div id="work-section">
  <div class="box">
    <h1>Hello!</h1>
  </div>
  <div class="box">
    <h1>Hello!</h1>
  </div>
  <div class="box">
    <h1>Hello!</h1>
  </div>
</div>