凹角

background-image: radial-gradient(circle at right top, whitesmoke, whitesmoke 10px, transparent 11px),
radial-gradient(circle at left top, whitesmoke, whitesmoke 10px, transparent 11px),radial-gradient(circle at right bottom, whitesmoke, whitesmoke 10px, transparent 11px),radial-gradient(circle at left bottom, whitesmoke, whitesmoke 10px, transparent 11px)