https://ja.wikisource.org/wiki/MediaWiki:Gadget-mathjax.js has the line
mw.loader.load('https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=MML_HTMLorMML-full');//load MathJax with a suitable combined config file

This shall be removed as it is third-party content.

(May need escalation to https://meta.wikimedia.org/wiki/Steward_requests/Miscellaneous#Please_remove_privacy_violating_site_CSS per T209998#6335220 or T208177#4703847.)