In the page namespace in Wikisource, there are tabs leading to the previous page, the next page and the related index page. They used to be indicated by arrows left, right and up respectively, but since the last MediaWiki train, they have instead shown the tab separator background instead.
The arrows are specified as CSS backgrounds on the tab's a element, and I believe this change by @Krinkle is what causes the breakage, since the tab separator is now specified on the same a element, overriding the CSS from ProofreadPage that sets the arrow backgrounds.
Possible solution: Add a span tag in the code for the ProofreadPage tabs, and add the arrow backgrounds to that element instead.