JSFiddle - React, Tailwind, and code Playground

HTML

<header>
  <svg class="oben" viewBox="0 0 2200 180">
    <path d="M-1,-1 L2200,-1 L2200,130 C1500,40,300,180,-1,100" style="fill-opacity: 0.85;" fill="#F0F0F0"></path> 
    <path d="M2200,130 C1500,40,300,180,-1,100" style="stroke-width: 5;" fill="rgba(0,0,0,0)" stroke="#22b222"></path> 
  </svg> 
  <nav>
    <a id="logoLink" href="#"><img src="daten/pics/logo_grau.png" width="230" height="100"></a>
    <ul class="navItems">
    <li><a href="#" title="...">Tätigkeitsschwerpunkte</a></li>
    <li><a href="#" title="...">Vita</a></li>
    <li><a href="#" title="...">Office</a></li>
    <li><a href="#" title="...">Terminsvertretung</a></li>
    </ul>
  </nav>
</header>
<!-- Header - ende -->
<main>
<!-- Mitte - Anfang -->
  <div class="mitte"> 
    <svg class="mitte" viewBox="0 0 2200 130">
      <path d="M2200,33 C1500,-57,300,83,-1,3" style="stroke-width: 5;" fill="rgba(0,0,0,0)" stroke="#c2c222"></path> 
      <path d="M2200,33 C1500,-57,300,83,-1,3 v127 L2200,130 Z" style="" fill="#ffffff"></path>
    </svg>
    <img id="schwerin" src="daten/pics/HeaderBilder/schwerin1.jpg" width="100%">        
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  </div>  
</main>

CSS

body { margin:0 auto; padding:0; font-size:100.01%; }

header { position:fixed; width:100%; top:0px; display:block; border: 2px solid green;}

header .oben { z-index:-10; position:absolute; }

nav { display: flex; }
#logoLink { flex: 0 0 230px; }
.navItems {
   list-style: none;
   padding: 0; margin: 1vw 0 0 0;
   flex: 1 1 auto;
   display: flex; flex-wrap: wrap; justify-content: center;
}
.navItems li:nth-of-type(n+2) { margin-left: 15px; }

a {
   color:#000;
   font-family: Arial, sans-serif; font-size: 18px; font-weight: bold;
   text-decoration: none;
}
a:hover {
  color: #b22222;
  text-decoration: none; }

main { margin-top: 5vw; }

.mitte { z-index:-10; position:relative; }