Viewing https://commons.wikimedia.org/wiki/Main_Page or https://en.wikipedia.org/wiki/Elizabeth_II and various pages on other wikis I often get this error in the console as of late:
Uncaught TypeError: Cannot read property 'insertRule' of undefined
Whenever this happens, the JavaScript runtime is broken very early on. So things like search suggestions, table of contents, sortable tables, centralnotice banners etc all do not operate.
I get this (blocking) error on any page on enwiki with an .ogg file on it. Chrome does not like somthing about the embedded player (that replaces the native player).
It's interesting that this suddenly started occurring...
Really, we should dump that entire spinner in favor of our core spinner module, but I'm wary about investing too much effort into it.
Change 255804 had a related patch set uploaded (by TheDJ):
Use mediawiki.util to insert style tags
Change 256446 had a related patch set uploaded (by Krinkle):
Use mediawiki.util to insert style tags
I wonder if this issue was the result of async loading and is thus also present in 1.26....