Reported in Chrome DevTools console when visiting https://doc.wikimedia.org/oojs-ui/master/demos/?page=widgets&theme=wikimediaui&direction=ltr&platform=desktop:
```
widgets.js:2835 Uncaught TypeError: (intermediate value).addItemsFromData is not a function
at Object.Demo.static.pages.widgets (widgets.js:2835)
at Array.<anonymous> (demo.js:299)
at fire (jquery.js:3268)
at Object.fireWith [as resolveWith] (jquery.js:3398)
at HTMLLinkElement.<anonymous> (jquery.js:3781)
at fire (jquery.js:3268)
at Object.fireWith [as resolveWith] (jquery.js:3398)
at HTMLLinkElement.deferred.(/oojs-ui/master/demos/anonymous function) (https://doc.wikimedia.org/oojs-ui/master/demos/node_modules/jquery/dist/jquery.js:3736:36)
at HTMLLinkElement.dispatch (jquery.js:5183)
at HTMLLinkElement.elemData.handle (jquery.js:4991)
```
This error is serious enough that it prevents rendering the page content, resulting only in an empty page with the navigation bar at the top.
Also the PHP demo results in a HTTP 500 error: https://doc.wikimedia.org/oojs-ui/master/demos/demos.php.
So far only the Dialogs, Icons and Toolbars sections of the JavaScript Demo seem to work.