The page here: https://cy.wiktionary.org/wiki/MediaWici:Monobook.js
was reported by that wiki's relevant Special Page - https://cy.wiktionary.org/wiki/Arbennig:LintErrors/missing-end-tag as containing a missing/mismatched tag.
I've checked the relevant page and the 'missing' tag is inside a Javascript Comment which should otherwise be ignored.
Confusing, but not unexpected.
Hmm .. this may not be sufficient. Mediawiki:Commons.css seems to have the wikitext content model.
Where are you getting that from?
https://en.wikipedia.org/wiki/Mediawiki:Commons.css?action=info
Page content model CSS
Change 352634 had a related patch set uploaded (by Arlolra; owner: Arlolra):
[mediawiki/services/parsoid@master] T163549: Only lint pages that have wikitext contentmodel
Change 352634 merged by jenkins-bot:
[mediawiki/services/parsoid@master] T163549: Only lint pages that have wikitext contentmodel
You should not lint module pages with wikitext contentmodel. At Wikivoyage/de bogus-image-options were given for lua source code.
https://de.wikivoyage.org/wiki/Spezial:LintErrors/bogus-image-options
It seems that there is the same with missing-end-tag and stipped-tag.
That patch assures that we only lint pages with the wikitext content model.
Generally, though, we close tasks once a fix has been merged, as above. However, it's yet to be deployed.
This requires a Parsoid deployment, not the Linter extension.
Parsoid deployments can be tracked here, https://www.mediawiki.org/wiki/Parsoid/Deployments
It most likely won't go out until Monday. Sorry
After today's deploy, these entries will no longer be generated, but existing db entries will continue to show up in the Special:LintErrors page. @Legoktm we should see if there is a simple way to purge the db of these entries.