The second time I clicked on the link, everything looked fine for me?

Hah, yeah, I experienced the same thing as @Elitre. The first time it looked as if none of the OOjs UI styles were loaded (but all the JavaScript code was loaded and working). It was probably intermittent, but maybe somebody has any idea of recent ResourceLoader changes that could result in bugs like this?

(Or something in that's site common.js that messes stuff up?)

I can consistently reproduce this by running localStorage.clear(); and then refreshing the page.

This is not caused by a change in ResourceLoader, however.

Rather, it's due to the site stylesheet having broken a syntax error in MediaWiki:Vector.css which broke some of the other stylesheets loaded in the same batch. Without cache, the time between batches is longer so they end up in a separate batch. When loading from cache they usually end up in the same batch.

Fixed in eo.wikipedia.org Revision #5789263.

See also T112552.

Thank you @Krinkle!