Since today, there appears to be an issue with the styling of several wikis, including enwiki and meta. Even though all of the content of the wiki page is visible on the screen normally, the actual page is exceptionally wide in the browser. There is a horizontal scrollbar visible, which scrolls the entire page. This occurs even though there is no visible content "right" of the page.
The actual HTML <body> element is showing as 1903px x 997px in Chrome Dev Tools, however, the actual page is some thousands of pixels wide. There is no obvious culprit in the HTML that is causing this. This occurs on all pages, so it isn't a formatting bug in the wikicode, and it occurs on both Firefox and Chrome. My default skin is Monobook.
This is creating a bad UI/UX combination, as the user is being led to believe that there is additional content to the right of the page, even when there is not.