transition

transition（过渡）
用法：

<div class="app">hello</div>
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.app {
  height: 100px;
  width: 100px;
  background: green;
  color: #ccc;
  text-align: center;
  line-height: 100px;
  transition: all 2s;
  transition-timing-function: ease-in-out;  //控制变化的均匀程度
  transition-delay: 1s;  //延迟几秒后才执行动画
}
.app:hover {
  width: 200px;
  background: red;
  color: yellow;
}