引入animate.css

<div id="app">
        <input type="button" value="toggle" @click="flag=!flag">
        <transition enter-active-class="animated bounceIn" leave-active-class="animated bounceOut">
            <h3 v-if="flag">这是一个H3</h3>
        </transition>
</div>
//或者下面这种形式
<div id="app">
        <input type="button" value="toggle" @click="flag=!flag">
        <transition enter-active-class="bounceIn" leave-active-class="bounceOut">
            <h3 v-if="flag" class="animated ">这是一个H3</h3>
        </transition>
</div>