#key - An angle in pureCSS
div {
	--angle: 150deg; /* CHANGE THIS VALUE TO THE ANGLE YOU WANT */
}

div {
    height: 300px;
    width: 600px;
    overflow: hidden;
    position: relative;
    background: crimson;
    border-radius: 300px 300px 0 0;
    border: 1px solid crimson;
}
div::before {
    height: inherit;
    width: inherit;
    position: absolute;
    content: "";
    background-color: white;
    transform-origin: 50% 100%;
    transform: rotate(var(--angle));
}