How do I add negative radius to half circle? - html

Question

I want to do this:
So far I got this:
.header {
position: fixed;
top: 0;
left: 0;
width: 100%;
background: #red;
z-index: 10000;
height: 10px;
overflow: visible;
}
.header:after {
content: '';
position: relative;
top: 100%;
display: block;
margin: 0 auto;
background: red;
width: 50px;
height: 25px;
border-radius: 10px 10px 50px 50px;
}
<div class="header"></div>
Codepen.
I can't get the top radius to go outwards the half circle like in the image.
How to do this with CSS?

Answer 1

You cannot make a negative radius on a border.
There is the possibility to make an SVG path or radial gradient... I made a new div as circle and radial gradient on pseudo-elements. It's not perfect, but it will possibly show you the direction to solution :)
.header {
position: fixed;
top: 0;
left: 0;
width: 100%;
background: red;
z-index: 10000;
height: 10px;
overflow: visible;
}
.header-circ {
position: relative;
top: 100%;
display: block;
margin: 0 auto;
background: red;
width: 500px;
height: 250px;
border-radius: 10px 10px 250px 250px;
}
.header-circ::before, .header-circ::after {
content: "";
position: absolute;
width: 100px;
height: 100px;
z-index: -1;
}
.header-circ::before {
left:-94px;
background: radial-gradient(circle at bottom left, white 0%,white 75%,red 75%);
}
.header-circ::after {
right:-94px;
background: radial-gradient(circle at bottom right, white 0%,white 75%,red 75%);
}
<div class="header"></div>
<div class="header-circ"></div>