I'm not asking everything in HK to be retagged as yue.

The name in standard written Chinese "書面語" in Hong Kong is basically the name used in BOTH Mandarin and Cantonese. There's no reason for us to introduce `cmn` for Mandarin in Hong Kong while using `zh` for Mandarin accross the globe and other websites/applications.

This is the same practice that the Taiwanese/Taigi language (Taiwanese Hokkien) would simply use nan-* instead of tagging them under zh (example: zh-Hant=板橋, nan-Hant=枋橋).

For the `Prefix: yue`, it means the language subtag of the language tag should be `yue` rather then specifying the "immediate-prefix".

Also, I'm pretty sure we need compatibility for Wikimedia/MediaWiki at least, with the fact that `zh-cmn` is being marked as deprecated and both `cmn` and `zh-cmn` is not commonly used both anywhere across the internet and in OpenStreetMap entities (nodes/ways/relations).