<div id="comparison">
    <figure>
        <div id="divisor"></div>
    </figure>
    <input type="range" min="0" max="100" value="50" id="slider" oninput="moveDivisor();">
</div>

div#comparison { 
    width: 60vw;
    height: 60vw;
    max-width: 600px;
    max-height: 600px;
    overflow: hidden;
    position: relative;
}

div#comparison figure { 
    background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/photoshop-face-before.jpg);
    background-size: cover;
    font-size: 0;
    width: 100%;
    height: 100%;
    margin: 0;
}

div#comparison figure > img {
    position: relative;
    width: 100%;
}

div#comparison figure div { 
    background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/photoshop-face-after.jpg);
    background-size: cover;
    position: absolute;
    width: 50%;
    box-shadow: 0 5px 10px -2px rgba(0,0,0,0.3);
    overflow: hidden;
    bottom: 0;
    height: 100%;
}

input[type=range]{
    -webkit-appearance:none;
    -moz-appearance:none;
    position: relative;
    top: -2rem;
    left: -2%;
    background-color: rgba(255,255,255,0.1);
    width: 102%;
}

input[type=range]:focus { 
    outline: none;
}

input[type=range]:active {
    outline: none;
}

input[type=range]::-moz-range-track {
    -moz-appearance:none;
    height:15px;
    width: 98%;
    background-color: rgba(255,255,255,0.1);
    position: relative;
    outline: none;   
}

input[type=range]::active {
    border: none;
    outline: none;
}

input[type=range]::-webkit-slider-thumb {
    -webkit-appearance:none;
    width: 20px;
    height: 15px;
    background: #fff;
    border-radius: 0;
}

input[type=range]::-moz-range-thumb {
    -moz-appearance: none;
    width: 20px;
    height: 15px;
    background: #fff;
    border-radius: 0;
}

input[type=range]:focus::-webkit-slider-thumb {
    background: rgba(255,255,255,0.5);
}

input[type=range]:focus::-moz-range-thumb {
    background: rgba(255,255,255,0.5);
}

divisor = document.getElementById('divisor');
slider = document.getElementById('slider');

function moveDivisor() {
    divisor.style.width = slider.value + '%';
}

window.onload = function() {
    moveDivisor();
};