HTML <div id="square"></div> CSS #square { width: 100px; height: 100px; background: blue; } Diamond: ダイアモンド HTML <div id="diamond"></div> CSS #diamond { width: 80px; height: 80px; background: blue; margin: 3px 0 0 30px; -webkit-transform: rotate(-45deg); -moz-transform: rotate(-45deg); -ms-transform: rotate(-45deg); -o-transform: rotate(-45deg); transform: rotate(-45deg); -webkit-transform-origin: