I know it's my fault and probably obvious but that site doesn't load on Chrome if you have disabled third-party cookies. You might want to show an informative message instead of an infinite spinner :(
Could you build a "show me what this would look like on my site" feature that takes their site, and fakes a version on your server (maybe in an iframe or using javascript)?
We've heard that "small side thing" a few times before so we'll fix that soon.
Happy to answer any other questions on our intercom chat!