Validator reports as error the '_self' value of 'target' attribute

[hugronaphor]

The error:
The attribute 'target' in tag 'a' is set to the invalid value '_self'.

The markup:
<a href="http://www.theweek.co.uk/election-2015" target="_self">UK general election</a>

This attribute value should be allowed even if this is a defualt value for 'target' attribute.

[cramforce]

The issue is that _self may be wrong in some embedding scenarios. This is why AMP pages should never rely on the default actually meaning _self. Allowing it would mean that embedders would have to explicitly do something else than the page is stating, which seems problematic.

[hugronaphor]

Ok, in this case we really need to push on Issue #994 as it's hard to validate the markup.