We had several reports of CSS being broken. Apparently happened over european night. Examples:
T138578: [Regression] en.wikipedia.org Mobile main page no longer being transformed
T138537: checkuserwiki doesn't have any skin
T138579: nl.wikisource CSS loading is broken
For the three first examples, appending ?debug=true to by pass the CSS/JS cache fix the rendering.
Best candidate is the RL site.styles which, when included, breaks the whole layout/stylesheet.