.main {
    background: url("bg.jpg") no-repeat;
    background-size: cover;
}

.mask {
    background-color: rgba(0,0,0,0.4);
}

.fade-enter-active, .fade-leave-active {
    transition: opacity .5s;
}
.fade-enter, .fade-leave-to /* .fade-leave-active below version 2.1.8 */ {
    opacity: 0;
}