Yes it does - and that way you get rid of part of the CSS definitions. In fact that mapping I’m taking about is not only for this purpose, but for greater good - moving “languages” into variants.
Also, html-lang attribute has other uses, e.g. it is important for SEO or accessibility (screen readers).
I always hated CSS manipulation of texts. If I want something to be shown in CAPITALS, I write it like that, I don’t want it decided by a template/web designer.
Actually, IANA/ISO/etc - thus CSS - does not cover all 7000+ languages, not all languages have lower/upper case, and CV is dealing with non-institutional languages.
IMHO, CV should not use text-transform at-all.