<div id="header"></div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 100">
  <path fill="green" d="M 0 0 Q 500 100 1000 0 Z"></path>
</svg>

#header {
  width: 100%;
  height: 100px;
  background-color: green;
}