I used the Random article link the the English Wikipedia sidebar to arrive at https://en.wikipedia.org/wiki/Sisters_of_the_Living_Word. The page loads with only very basic CSS.

The CSS request that is failing specifically is a GET request to https://en.wikipedia.org/w/load.php?lang=en&modules=ext.cite.styles%7Cext.echo.styles.badge%7Cext.uls.interlanguage%7Cext.visualEditor.desktopArticleTarget.noscript%7Cext.wikimediaBadges%7Cmediawiki.legacy.commonPrint%2Cshared%7Cmediawiki.skinning.interface%7Cmediawiki.toc.styles%7Coojs-ui.styles.icons-alerts%7Cskins.vector.styles%7Cwikibase.client.init&only=styles&skin=vector.

On Safari 13.0.2 (15608.2.30.1.1) running on macOS 10.15 when I am logged into Wikipedia the above request fails with a Protocol error: NXPOSIXErrorDomain: 100. I have tried in both normal and Incognito mode, and have tried using two separate Wikipedia accounts, one of which has all default display settings.

When I am not logged in the CSS document loads as expected. Even manually requesting the above URL while not logged in returns a valid CSS document.

On Google Chrome 78.0.3904.97 all CSS loads as expected, whether I am logged in or not.

So far I have only experienced this bug on https://en.wikipedia.org/wiki/Sisters_of_the_Living_Word. Even that article's talk page displays fine for me, CSS and all.