If you open any article, it becomes horizontally scrollable for no discernible reason. If you scroll horizontally, it's just empty space to the right of the text.
Interestingly, if you scroll down far enough, the horizontal scrollability disappears, and the article becomes normal again.
For example, open the following URL in a shrunken-down browser window:
https://en.wikipedia.org/api/rest_v1/page/mobile-html/Cat