Compare Two Images HTML

I am following instructions on https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_image_compare, but when I place it on cs-cart, it becomes like this https://prnt.sc/1s45zwx. And, I would like to make it works similar to this https://htsclinic.com/results/.

I am not sure about the issue with cs-cart but it seems like the javascript is not working.