Small update. I did some digging and I assume the display is using the IE engine which in my case is IE11. When I open the startpage.html directly in IE11 I get several error messages in the console one specifically:
CSS was ignored due to mime type mismatch (main.css)
CSS was ignored due to mime type mismatch (jquery.fancybox-1-3-4.csss)
Script438: Object doesn't support property or method 'GetMRU1'
File: startpage.html, line 171, column 6
Does anyone know how to fix this. I've tried resetting my IE11 settings (I don't use the browser anyway). Opening the page in Chrome displays it correctly.