.fade-enter-active, .fade-leave-active {
transition-property: opacity;
transition-duration: .25s;
}

.fade-enter-active {
transition-delay: .15s;
}

.fade-enter, .fade-leave-active {
opacity: 0
}

<transition name="fade">
<router-view name="main" :key="$route.fullPath"></router-view>
</transition>