It is not recommended to have nested <section>. A better approach would be to replace your outer <section> with
<div class="wrapper">...</div>
Next, utilize the wrapper class to apply a margin-top: 54px, which corresponds to the height of the navigation.
.wrapper {
margin-top: 54px;
height: 100vh;
background: #303030;
padding: 0px;
}
To address the overlap issue, use the calc function to dynamically set the height like so: height: calc(100vh - 52px);
*,
*::after,
*::before {
margin: 0px;
padding: 0px;
box-sizing: inherit;
-webkit-box-sizing: inherit;
}
body,
html {
margin: 0px;
padding: 0px;
box-sizing: border-box;
-webkit-box-sizing: border-box;
}
nav {
position: fixed;
top: 0px;
left: 0px;
width: 100%;
text-align: center;
padding: 10px 0px;
background: tomato;
}
nav ul {
margin: 0px;
padding: 0px;
list-style: none;
text-align: center;
}
nav ul li {
display: inline-block;
margin: 3px;
}
nav ul li a {
display: block;
padding: 5px;
background: #000;
color: #ddd;
text-decoration: none;
}
nav ul li a:hover {
background: #ddd;
color: #333;
}
section {
height: 100vh;
background: #303030;
margin: 0px;
padding: 0px;
}
.wrapper {
margin-top: 54px;
height: 100vh;
background: #303030;
padding: 0px;
}
section p {
color: #fff;
}
section.green {
background: green;
height: calc(100vh - 52px);
margin: 0px;
padding: 0px;
}
section.green p {
color: #ddd;
}
<div class="wrapper">
<nav>
<ul>
<li><a href="javascript:void(0);">Home</a></li>
<li><a href="javascript:void(0);">About</a></li>
<li><a href="javascript:void(0);">Service</a></li>
<li><a href="javascript:void(0);">Contact</a></li>
</ul>
</nav>
<section>
<p>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>
</section>
<section class="green">
<p>
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>
</section>
</div>