I have now found references to PURPOSE but...
Which does google actually use?
The correct form is HTTP_X_PURPOSE but webkit seems to advocate HTTP_X-PURPOSE (which I've never seen before). Since chrome is webkit I assume they are using that version. Certainly that is the case in the google reference document itself.
I assume the prefix HTTP is correct, or is google also breaking that?
I still can't believe the arrogance of blacklisting a web site based on the results of a possibly illegal prefetch!
And the idea that "most web pages will load as soon as the URL has been typed into the address bar, before the user hits Enter" implies that a) google prefetches a LOT of pages whilst the punter is still typing and b) that all of those web sites are on very fast servers, in which case, why bother prefetching?