The extension's page on MediaWiki.org states the following:
The category may also be added, and the content will not be highlighted, if there are more than 1000 lines or more than 100 kB text.
While this behaviour is true for the syntaxhighlight tag, it is not for CSS and JS pages, where highlighting is simply not invoked if the byte limit condition isn't met and no indication is left beside, obviously, the lack of highlighting. Code pages shouldn't be exempted.
- Tracking for pages affected by T104109
- Indication the syntax highlighter didn't run due to an error and not just on a whim.