SVG
As Harry suggested in the comments, SVG would be your best option as a double curve in CSS isn't feasible without using multiple elements, pseudo elements or using possibly unsupported CSS3 properties.
SVG stands for Scalable Vector Graphic. The web browser views it as an image but you can add text and normal HTML elements within an SVG.
It is well supported across all browsers as viewable here: CanIUse
<svg width="466" height="603" viewbox="0 0 100 100" preserveAspectRatio="none">
<path d="M0,0
L100,0
C25,50 50,75 0,100z" fill="#8aa7ca" />
</svg>
CSS
Ofcourse this is still possible with CSS but does take using pseudo elements :before
and :after
. It is also not best for the curves as it will render them without anti-aliasing
div {
background: blue;
width: 50px;
height: 75px;
position: relative;
}
div:before {
content: '';
background-image: radial-gradient(circle at 100% 100%, rgba(204, 0, 0, 0) 100px, blue 100px);
position: absolute;
top: 0;
left: 100%;
width: 100px;
height: 75px;
}
div:after {
content: '';
position: absolute;
width: 50px;
height: 75px;
background: blue;
border-radius: 0 0 100% 0 / 0 0 100% 0;
top: 100%;
left: 0;
}
<div></div>