I fixed it here: https://jsfiddle.net/nv5s694h/
Thank you for helping me understand it better.
<div class="outer">
.outer is the parent element to the .container elements.<br><br>
<div class="container play1 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain1">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play2 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain2">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play3 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain3">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play4 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain4">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play5 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain5">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play6 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain6">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play7 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain7">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play8 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain8">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
<br><br>
<div class="container play9 with-curtain">
.container elements are siblings within .outer<br>
.container is the parent to inner-container<br><br>
<div class="inner-container curtain curtain9">
.inner-container is the parent to .ratio-keeper<br><br>
<div class="ratio-keeper">
ratio-keeper, which is parent to .wrap and .sliding-panels<br><br>
<div class="wrap">
.wrap, which is the parent to video<br><br>
<div class="video video-frame" data-id="CHahce95B1g">
video, which is the child of .wrap
</div>
<br>
</div>
<div class="sliding-panels">
.sliding-panels, which is the parent to .panel-left, .panel-right
<br> <br>
<div class="panel-left">
.panel-left, which is the child of .sliding-panels
</div>
<br>
<div class="panel-right">
.panel-right, which is the child of .sliding-panels
</div>
</div>
</div>
</div>
</div>
</div>