css实现马赛克画板

background: linear-gradient(45deg,rgba(0,0,0,.2) 25%,transparent 0,transparent 75%,rgba(0,0,0,.2) 0),
            linear-gradient(45deg,rgba(0,0,0,.2) 25%,transparent 0,transparent 75%,rgba(0,0,0,.2) 0);
            background-size: 30px 30px;
            background-position: 0 0,15px 15px;