結論からいえば、animation-fill-modeプロパティを forwards にすればOKです。
DEMO
CSS
.box { position: absolute; top: 0; margin: 10px; width: 0; height: 0; background: #3e3e3e; box-shadow: 0 0 10px rgba(0, 0, 0, .25); } .b1 { left: 0; animation: anim 2.5s ease-in-out .5s; } .b2 { left: 110px; animation: anim 2.5s ease-in-out .5s forwards; } @keyframes anim { 0% { width: 0px; height: 0px; } 100% { width: 100px; height: 100px; } }