This is a script. addEventListener('DOMContentLoaded', function () was executed on the document. document.querySelectorAll('img.thumb-img[data-preview]') is something that you should do.a function called forEach (image) When the previewSrc variable is set to img.getAttribute('data-preview'), the following code is executed: if (previewSrc) { img.addEventListener('mouseenter', function () { const video = document.createElement('video')