In addition, I am firmly convinced that the renderers used in browsers will be able to synthetize themselves the flags according to their wellknow ISO 31166-1 codes, in absence of font support: this will just require for them to ship a small collection of SVG graphics (something that is already widely available). This will be valid substitution immediately, in absence of a more general technology based on an external registry, and of support in fonts.
The technical needs for developping it in renderer software is very small. It will also be easy to test as there's no complication.