I made a comment at T101246#4062458, from which I can quote (but made tweaks).
> The feature works at the 2003/default WikiEditor (the one without the javascript-coded toolbars filled with buttons as seen in later WikiEditors). I just have to:
> # Enable the Toolbar __in [[https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-editing|Preference settings]] __
> # Enable the "Syntax Highlighting" feature in the [[https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-betafeatures|Beta Preference]]
> # Click the syntax highlighting button in the Toolbar
> # Disable the Toolbar without disabling the feature
I originally discussed this at https://meta.wikimedia.org/wiki/Talk:Community_Tech/Wikitext_editor_syntax_highlighting#Not_working_in_2003_WikiEditor_and_whether_to_enable_or_disable_by_default . How do you allow directly enabling or disabling the feature in the 2003 WikiEditor without needing to enable the Toolbar to click on the "Highlighter" icon button? If [[https://www.mediawiki.org/wiki/User:Remember_the_dot/Syntax_highlighter|Remember the dot's Syntax highlighter]] feature can be directly opt-in/opt-out without a Toolbar, why not this other feature?