SyntaxHighlight_GeSHi should auto-populate a tracking category with pages that contain invalid <syntaxhighlight> attributes such as lang="html9". This will allow editors to find and address these pages by setting lang="text" or by filing Phabricator Maniphest tasks to add support for missing languages.
Related: https://lists.wikimedia.org/pipermail/wikitech-l/2015-June/082165.html
Change 221311 had a related patch set uploaded (by Ori.livneh):
Add a tracking category on pages that contain invalid <syntaxhighlight> attributes
Change 221311 merged by jenkins-bot:
Add a tracking category on pages that contain invalid <syntaxhighlight> attributes
We should probably announce to people that the tracking category: Pages with syntax highlighting errors, is now added to pages with syntax highlighting errors.
Where can I find a list of valid values for the lang= attribute? Previously, it was possible to get a list by entering some crud value like lang="sdyvgsuydfg" and previewing; an error would be thrown, and you could click a link for a list.
Tech News: 2015-29 says: You can now see a list of pages with errors in code coloring. [4] . On nl-wiki the link to the list of pages is still red. Should we do something to fill that category?
Categories do not need to 'exist' to contain pages. However, a description can be helpful to newbies, even if it's empty.
This tracking category also gets added to a page if the syntax highlighting fails due to the input being larger than 102,400 bytes (cf. T104109).