On the live plantUML server I'm running into an issue with UXP browsers that don't define the (ambiguous) global "event". As a result the entire site isn't working.
This seems to be caused by a standalone bit of scripting for token checking/generation.
<script>var __ez_tkn_evnt = function() { if(typeof(_ezaq) != 'undefined'){if(window.ezogtk != "") {__ez.bit.AddAndFire(_ezaq.page_view_id,[new __ezDotData("ext_user_hash",window.ezogtk)]);}}};document.addEventListener("DOMContentLoaded", __ez.queue.addFunc("__ez_tkn_evnt", "__ez_tkn_evnt", event, false, ['/detroitchicago/minneapolis.js','/detroitchicago/memphis.js'], true, true, false, true));</script>
The solution would be to pass the event into the event handler function instead of relying on the ambiguous window.event alias, and using that.