May be a result of onwiki changes to https://meta.wikimedia.org/wiki/API_listing_template
Steps to reproduce:
Visit /api/ on a wiki, eg https://en.wikipedia.org/api/ or https://meta.wikimedia.org/api/
Console warning:
Refused to apply style from 'https://meta.wikimedia.org/api/rest_v1/?path=css/reset.css' because its MIME type ('application/problem+json') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
Refused to apply style from 'https://meta.wikimedia.org/api/rest_v1/?path=css/screen.css' because its MIME type ('application/problem+json') is not a supported stylesheet MIME type, and strict MIME checking is enabled.