I would only think that's the case when the language of the file you're editing is unknown, so you would have to do some server-side analysis of the entire file

I would only think that's the case when the language of the file you're editing is unknown, so you would have to do some server-side analysis of the entire file and make an educated guess. For example, https://github.com/github/linguist https://github.com/github/linguist. But if the language is known in advance, then you could send the grammar rules to the client.