.panel-outer {
    position: relative;
    white-space: nowrap;
    /* perspective: 500px; */
    background-color: #222;
}

.panel-inner {
    height: 50%;
    width: 100%;
    position: absolute;
    overflow: hidden;
}

.panel-upper {
    transform-origin: bottom;
}
.panel-upper::before {
    content: '';
    height: 2%;
    width: 100%;
    position: absolute;
    bottom: 0;
    background-color: #000;
    opacity: 0.8;
}

.panel-upper::after {
    content: '';
    height: 100%;
    width: 100%;
    position: absolute;
    top: 0; left: 0;
    background: linear-gradient( 
        #fff 0%, 
        #999 50%,
        #000 98%, 
        #000 100%);
    opacity: 0.1;
}

.panel-lower {
    top: 50%;
    transform-origin: top;
}
.panel-lower::before {
    content: '';
    height: 2%;
    width: 100%;
    position: absolute;
    top: 0;
    background-color: #000;
    opacity: 0.8;
}
.panel-lower::after {
    content: '';
    height: 100%;
    width: 100%;
    position: absolute;
    top: 0; left: 0;
    background: linear-gradient( 
        #fff 0%, 
        #999 50%,
        #000 98%, 
        #000 100%);
    opacity: 0.1;
}

.panel-lower > div {
    transform: translateY(-50%);
}

@keyframes rotate-0-upper {
    0% { transform: rotateX(0deg); filter: brightness(100%); }
    100% { transform: rotateX(-90deg); filter: brightness(25%); }
}
.rotate.panel-0-upper { animation: rotate-0-upper 0.1s linear both; }

@keyframes rotate-1-lower {
    0% { transform: rotateX(90deg); filter: brightness(300%); }
    100% { transform: rotateX(0deg); filter: brightness(100%); }
}
.rotate.panel-1-lower { animation: rotate-1-lower 0.1s linear both 0.1s; }

.none { display: none; }

.panel-1-upper,
.panel-0-lower {
    z-index: 95;
}
.panel-1-upper::before,
.panel-0-lower::before {
    z-index: 96;
}
.panel-1-upper::after,
.panel-0-lower::after {
    z-index: 97;
}

.panel-0-upper,
.panel-1-lower { 
    z-index: 100;
}
.panel-0-upper::before,
.panel-1-lower::before {
    z-index: 101;
}
.panel-0-upper::after,
.panel-1-lower::after {
    z-index: 102;
}