clip-path curvness
HTML
<svg width="0" height="0">
<defs>
<clipPath id="myClip" clipPathUnits="objectBoundingBox">
<path d="M0,0 1,0 1,0.9 C 1,0.9, 0.77,1, 0.5,1 0.23,1, 0,0.9,0,0.9z"/>
</clipPath>
</defs>
</svg>
<header></header>
CSS
header {
background: black;
min-height: 150px;
clip-path: url(#myClip);
}