I'm trying to add HTML code I found on CodePen for a hexagonal grid. I've added the CSS without a problem. However, when I add the HTML to the same code box in Upfront, I get an error that shows with the red exclamation mark that there is a problem with the code. But I don't know what the problem is that Upfront isn't allowing this to work. I'm copying the code directly from the CodePen site and it is open sourced on Git Hub as well.
I'm just starting to rebuild my site, so there is nothing there right now. I've opened the site to support. I'd appreciate help to find out how I can implement this code. Thank you.