JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container cf">
    
    <div class="kopf"><h1>Kopfbereich</h1></div>
        
    <div class="navigation">
        <ul>
            <li><a href="#">Über uns</a></li>
            <li><a href="#">Service</a></li>
            <li><a href="#">Kontakt</a></li>
        </ul>    
    </div>
    
    <div class="inhalt">
        <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
        <p><img src="http://placehold.it/525x350" alt="Demo Grafik"></p>
        <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
        <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy...

CSS

* {
    mox-box-sizing: border-box;    
    webkit-box-sizing: border-box;    
    box-sizing: border-box;
}

.cf:after {
    content: " ";
    display: table;
}

.cf:after {
    clear: both;
}

body {
    margin: 0;
    padding: 0;
    background-color: #43373F;
    color: #634C5C;
    line-height: 1.3;
    font-family: sans-serif;
    font-size: 1em;
}

img {
    max-width: 100%;
    height: auto;
}

.container {
    background: #efefef;
    width: 95%;
    margin: 0 auto;
    max-width: 980px;
}

.kopf {
    background-color: #48d1cc;
    color: #fff;
    padding: 1.875em;
}

.kopf h1 {
    font-weight: normal; 
    text-align: center;
}

.navigation {
    width: 37.5%;
    float: left;
    padding: 3.125%;
}

.inhalt {
    background-color: #fff;
    float: left;
    width: 62.5%;
    padding: 3.125%
}

a:link, a:visited {
    color: ce6634;
    text-decoration: none;
}

@media screen and (max-width: 400px) {
    
    .container {
        width: 100%;
    }
    
    .inhalt {
        text-align: justify;
         padding: 5.5%
    }
    
    .navigation, .inhalt {
        float: none;
        width: auto;
    }
    
    .navigation ul {
        padding-left: 0;
    }
    
    .navigation li {
        display: inline-block;
        list-style-type: none;
        padding: 0 3%;
    }

}