@Iniquity Considering it more carefully, I think this functionality is too difficult to implement and not very useful. Many templates that are not registered shouldn't be highlighted, like {{Dead link}}. How do we distinguish unregistered templates that should be highlighted from unregistered templates that shouldn't be highlighted, in a cross-wiki way? But even if we figured this out, what would be the point? Just aesthetic? Or is it to motivate users to register the templates and add their TemplateData? If so, the goal of the functionality would be to not use it. The goal would be that all templates are registered, so that the red highlight never appears. Does it make sense to implement a difficult functionality whose goal is not to be used?

@Sophivorus Yep, this is good question. But finally I wanted to separate BadTemplates, BareUrls and 'simple text', not just highlight unregestered templates.