Well, there is some Flash replacement code in many browser versions but current versions don't seem to be too affected. I am currently using Gecko v78.5.0 (Firefox/Waterfox) and Webkit 87.0.4280.66 (Google Chrome/Chromium) with no Flash plug-in and seems to be working fine.
Front-facing code is in the Lightbox section which is for popup containers:
<!-- lightbox 関連 を読み込む-->
<script type="text/javascript" src="lightbox2.04/js/prototype.js"></script>
<script type="text/javascript" src="lightbox2.04/js/scriptaculous.js?load=effects,builder"></script>
<script type="text/javascript" src="lightbox2.04/js/lightbox.js"></script>
<script type="text/javascript">
LightboxOptions.fileLoadingImage = "lightbox2.04/images/loading.gif";
LightboxOptions.fileBottomNavCloseImage = "lightbox2.04/images/cancel.png";
</script>
<script type="text/javascript" src="swfobject_2_2/swfobject.js"></script>