Load Wistia video after preview is clicked in an iframe. Modified from https://stackoverflow.com/a/47715361

markup.html

<!-- data-embed is wistia video id -->
<!-- extract data-thumb https://wistia.com/support/developers/extracting-thumbnails -->

<div class="wistia" data-embed="919q2r7d8r" data-thumb="6b08c24a857370a2a85a9b22539429340df8db0b">
    <div class="play-button"></div>
</div>

script.js

( function() {

var wistia = document.querySelectorAll( ".wistia" );

for (var i = 0; i < wistia.length; i++) {

var source = "https://embed-ssl.wistia.com/deliveries/"+ wistia[i].dataset.thumb +".jpg";

var image = new Image();
image.src = source;
image.addEventListener( "load", function() {
wistia[ i ].appendChild( image );
}( i ) );

wistia[i].addEventListener( "click", function() {

var iframe = document.createElement( "iframe" );

        iframe.setAttribute( "frameborder", "0" );
        iframe.setAttribute( "allowfullscreen", "" );
        iframe.setAttribute( "mozallowfullscreen", "" );
        iframe.setAttribute( "webkitallowfullscreen", "" );
        iframe.setAttribute( "oallowfullscreen", "" );
        iframe.setAttribute( "msallowfullscreen", "" );
        iframe.setAttribute( "src", "//fast.wistia.net/embed/iframe/"+ this.dataset.embed +"?videoFoam=true" );

        this.innerHTML = "";
        this.appendChild( iframe );
} );    
};

} )();

style.css

.wistia {
    background-color: #000;
    margin-bottom: 30px;
    position: relative;
    padding-top: 56.25%;
    overflow: hidden;
    cursor: pointer;
}
.wistia img {
    width: 100%;
    top: 0; /* Change this for different ratio than 16:9 */
    left: 0;
    opacity: 0.7;
}
.wistia .play-button {
    width: 90px;
    height: 60px;
    background-color: #333;
    box-shadow: 0 0 30px rgba( 0,0,0,0.6 );
    z-index: 1;
    opacity: 0.8;
    border-radius: 6px;
}
.wistia .play-button:before {
    content: "";
    border-style: solid;
    border-width: 15px 0 15px 26.0px;
    border-color: transparent transparent transparent #fff;
}
.wistia img,
.wistia .play-button {
    cursor: pointer;
}
.wistia img,
.wistia iframe,
.wistia .play-button,
.wistia .play-button:before {
    position: absolute;
}
.wistia .play-button,
.wistia .play-button:before {
    top: 50%;
    left: 50%;
    transform: translate3d( -50%, -50%, 0 );
}
.wistia iframe {
    height: 100%;
    width: 100%;
    top: 0;
    left: 0;
}