.flip:hover {
    transform: rotateY(180deg);
    transform-style: preserve-3d;    
}

.front{
    color: black;
}

.back{
    transform: rotateY(180deg);
}
.inner{
    position: relative;
    width: 100%;
    height: 100%;
    text-align: center;
    transition: transform 0.6s;
    transform-style: preserve-3d;
}

.front, .back {
    position: absolute;
    width: 100%;
    height: 100%;
    -webkit-backface-visibility: hidden;
    backface-visibility: hidden;
}