body{/*市松模様*/
background-size:40px 40px;
background-position:20px 20px,0 0;
background-image:
linear-gradient(45deg,#000 25%,transparent 25%,transparent 75%,#000 75%),
linear-gradient(45deg,#000 25%,#4b9 25%,#4b9 75%,#000 75%);
}