弧形切角

width:200px;

height:150px;

background:  radial-gradient(circle at bottom right, transparent 15px, aqua 0) bottom right,

　　　　　　radial-gradient(circle at bottom left, transparent 15px, aqua 0) bottom left,

　　　　　　radial-gradient(circle at bottom left, transparent 15px, aqua 0) bottom left,

　　　　　　radial-gradient(circle at bottom right, transparent 15px, aqua 0) bottom right;

background-size: 50% 50%;

background-repeat:no-repeat;