graeme_p - 6:09 am on Oct 23, 2012 (gmt 0)

I misunderstood this initially.

Lucy, competitors cannot really do that because it must be used on all pages (presumably it gets ignored if not). So if example.com/en has <link rel="alternate" hreflang="es" href="http://example.com/es" /> then example.com/es must have <link rel="alternate" hreflang="en" href="http://example.com/en" />

On the other hand, the scope for webmasters to mess this up seems quite large.....

I don't think g### distinguishes between "en" and "en-uk".

One of Google's scenarios for using this is for content with "small regional variations". They say "For example, you might have English-language content targeted at readers in the US, GB, and Ireland."

they did not mention header-level language specification

Yes, but I want to know! It seems to be to be the easiest way of doing it in most cases.