I started reading through the Fate Codex before the site revamp, and so I don’t believe this was an problem before. Many of the Fate Codex pages do not conform to screen sizes, with the text extending too far horizontally. As far as I can tell, this is mostly an issue on QuickStart adventure settings, which often have an image showing index cards with sample character and setting information. Some early Codex issues have these images, but they are rather small. In later issues, these images are incredibly large, which I believe is causing the problem. The issue covers also have this problem, but it isn’t as important as these pages are largely a table of contents, so the formatting isn’t changed.
In case this information is important, I was able to see this issue on desktop, mobile, and on a tablet.
A few examples (oops, I can only do two):
A QuickStart with images that does not have the problem is the Silicon City in codex 1 Issue 1.