There is a HTTP 500 at https://doc.wikimedia.org/oojs-ui/master/demos/ or really a subpage of that page.
Click on the JS-PHP switch to get the PHP version, and then you get
Our servers are currently under maintenance or experiencing a technical problem. Please try again in a few minutes.
See the error message at the bottom of this page for more information.
And at the bottom of the page
Request from 126.96.36.199 via cp1087 cp1087, Varnish XID 377456537
Error: 500, Internal Server Error at Thu, 13 Dec 2018 18:39:17 GMT