@keyframes heartbeat {
    0% {
        transform: scale(1.0);
    }

    33% {
        transform: scale(0.95);
    }

    66% {
        transform: scale(1.15);
    }

    100% {
        transform: scale(1);
    }
}

.modal50perc:hover .heartbeat_once {
    animation: heartbeat 0.6s;
    animation-iteration-count: 1;
}