Is 'en-EU' a valid value for HTML 'lang' attribute?

Question

Markup for different editions of my news site...

• US Edition:

   ... 
  

  <

Answer 1

Why not?

1. HTML5 (CR) defines that the "value must be a valid BCP 47 language tag, or the empty string".

2. BCP 47 defines that the "IANA Language Subtag Registry […] contains a comprehensive list of all of the subtags valid in language tags".

3. IANA Language Subtag Registry contains the subtag EU of type region.

It’s not up to anyone to formally define if or which linguistic variations are spoken in which regions. Language changes constantly, speakers live and die everywhere.