In my experience, I have not encountered this error - the content consistently appears behind the header in your provided example.
To prevent this issue, you can utilize "z-index", a CSS attribute that determines the stacking order of elements on a webpage.
In the scenario below, I assigned a z-index of 10 to your header, ensuring it remains in front of any elements lacking a z-index, such as the content.
<!DOCTYPE html>
<!--[if gt IE 8]><!-->
<html class="no-js">
<!--<![endif]-->
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<title>TitleT</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="description" content="" />
<meta name="keywords" content="" />
<meta name="author" content="" />
<!-- Facebook and Twitter integration -->
<meta property="og:title" content="" />
<meta property="og:image" content="" />
<meta property="og:url" content="" />
<meta property="og:site_name" content="" />
<meta property="og:description" content="" />
<meta name="twitter:title" content="" />
<meta name="twitter:image" content="" />
<meta name="twitter:url" content="" />
<meta name="twitter:card" content="" />
<!-- Place favicon.ico and apple-touch-icon.png in the root directory -->
<!-- <link rel="shortcut icon" href="favicon.ico"> -->
<link rel="stylesheet" href="css/animate.css">
<link rel="stylesheet" href="css/bootstrap.css">
<link rel="stylesheet" href="css/icomoon.css">
<link rel="stylesheet" href="css/owl.carousel.min.css">
<link rel="stylesheet" href="css/owl.theme.default.min.css">
<link rel="stylesheet" href="css/style.css">
<script src="js/modernizr-2.6.2.min.js"></script>
</head>
<body class="boxed">
<div id="wrap">
<div id="fh5co-page">
<style>
* {
margin: 0;
padding: 0;
}
#header {
background-color: #F9F6F3;
position: fixed;
top: 0;
left: 0;
width: 100%;
height: 100px;
z-index: 10;
}
</style>
<div id="header">
<header id="fh5co-header" role="banner">
<div class="container">
<a href="#" class="js-fh5co-nav-toggle fh5co-nav-toggle dark"><i></i></a>
<div id="fh5co-logo">
<a href="/"><img src="images/logo.png" height="50" width="95" alt=""></a>
</div>
<nav id="fh5co-main-nav" role="navigation">
<ul>
<li><a href="about">sdf</a></li>
<li class="has-sub">
<div class="drop-down-menu">
<a href="services.html">dfgdfg</a>
<div class="dropdown-menu-wrap">
<ul>
</ul>
</div>
</div>
</li>
<li><a href="portfolio.html">asd</a></li>
<li class="has-sub">
<div class="drop-down-menu">
<a href="#">gzu</a>
<div class="dropdown-menu-wrap">
<ul>
<li><a href="quotation">Ser</a></li>
</ul>
</div>
</div>
</li>
<li class="cta"><a href="contact">Contact</a></li>
</ul>
</nav>
</div>
</header>
</div>
<br>
<br>
<div id="fh5co-intro" class="fh5co-section">
<div class="container">
<div class="row row-bottom-padded-sm">
<div class="col-md-8 col-md-offset-2 text-center ts-intro">
<h1>About Us</h1>
</div>
</div>
<div class="row row-bottom-padded-sm">
<div class="col-md-8" id="fh5co-content">
<h2>History</h2>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Temporibus possimus, quaerat tenetur quos ad nostrum eaque esse, ex blanditiis deleniti accusantium molestias perferendis odio, iste cum. Hic eius, ullam repellendus. Possimus, ratione?
Qui, tempora, eveniet! Eveniet facilis nesciunt commodi nostrum!</p>
<p>Illum provident, rerum hic porro doloremque alias voluptatibus fugit aliquam aspernatur molestiae debitis fuga, expedita vel animi eius, ab natus in enim repellendus harum! Est atque quae aut? Sint quis expedita illo placeat ad saepe hic
quidem dolore laboriosam? Sapiente.</p>
<p>Similique sint consequuntur cum ea quia placeat aliquid blanditiis atque, et earum sit sequi. Earum dignissimos omnis, hic atque ducimus quibusdam eos, exercitationem cum ea ipsum, est animi nesciunt. Consequuntur molestias ratione asperiores
sequi nam minima dignissimos voluptas consequatur nemo.</p>
<p>Ut sit repellat id dignissimos excepturi recusandae reprehenderit eius, voluptatem et ab quos qui sequi rem sapiente, delectus numquam officiis. Maxime aliquam unde eum quo, sit qui voluptatum odio nam. Iste vitae, sed enim facilis suscipit
quis obcaecati quas, nobis.</p>
<p>Cupiditate commodi velit officia quidem dolores incidunt, deserunt nam quaerat aliquam labore iure quam. Exercitationem tempora itaque error odio quasi quas illo consequatur nisi debitis, aliquam magnam voluptas, quia, impedit, architecto
fugit suscipit officia ullam laudantium sit libero excepturi deserunt.</p>
</div>
<div class="col-md-4" id="fh5co-sidebar">
<div class="fh5co-service text-left">
<img src="images/slider_2.jpg" class="img-responsive img-bordered" alt="Free HTML5 Bootstrap Template by FreeHTML5.co">
<h3>WDesign</h3>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Pariatur ullam unde, totam harum laboriosam dolores.</p>
<ul class="ul_style_1">
</ul>
</div>
</div>
</div>
<div class="row row-bottom-padded-sm">
<div class="col-md-6">
<p><img src="images/slider_3.jpg" alt="Free HTML5 Bootstrap Template" class="img-responsive img-bordered"></p>
</div>
<div class="col-md-6 padded-top">
<h3>Creative Title Here</h3>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ipsum fuga rerum doloremque dolore, molestias eligendi. Nulla aliquam aut doloribus tenetur repellendus omnis dicta, unde magni.</p>
<ul class="ul_style_1">
</ul>
</div>
</div>
</div>
</div>
<footer id="fh5co-footer" role="contentinfo">
<div class="container">
<div class="row row-bottom-padded-sm">
<div class="col-md-4 col-sm-12">
<div class="fh5co-footer-widget">
<h3>About Us</h3>
</div>
</div>
<div class="col-md-3 col-md-push-1 col-sm-12 col-sm-push-0">
<div class="fh5co-footer-widget">
<h3>Quick Links</h3>
<ul class="fh5co-footer-link">
</ul>
</div>
</div>
<div class="col-md-3 col-md-push-2 col-sm-12 col-sm-push-0">
<div class="fh5co-footer-widget">
<h3>Follow us</h3>
<ul class="fh5co-social">
</ul>
</div>
</div>
</div>
<div class="row">
<div class="col-md-12">
</div>
</div>
</div>
</footer>
</div>
</div>
<center>
<center><small class='block'><font size='3'>© 2015-2018 by <a href='https://auszdgfuzasdfg.com'></font></small></center>
</center>
</body>
</html>