I'm trying to recreate this image with CSS:
I would not need it to repeat. This is what I started but it just has a straight line:
I think this is the right way to make a shape like you want. By using the SVG possibilities, and an container to keep the shape responsive.
svg {
display: inline-block;
position: absolute;
top: 0;
left: 0;
}
.container {
display: inline-block;
position: relative;
width: 100%;
padding-bottom: 100%;
vertical-align: middle;
overflow: hidden;
}
<div class="container">
<svg viewBox="0 0 500 500" preserveAspectRatio="xMinYMin meet">
<path d="M0,100 C150,200 350,0 500,100 L500,00 L0,0 Z" style="stroke: none; fill:red;"></path>
</svg>
</div>