css 2

三角形

width: 0;
height: 0;
border-top: 50px solid red;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-bottom: 100px solid blue;

弧形尾巴

width: 100px;
height: 100px;
background: radial-gradient(
  circle at 0 0,
  rgba(18, 105, 255, 0.798) 110px,
  #ce0000 100px
);