...
"The HTML representation of the URI "about:blank" MUST use the origin
and the effective script origin as defined by HTML5 Section 5.4
Origin [W3C.WD-html5]"
but the HTML representation of the URI "about:blank"
is not defined as something that has an origin or
an effective script origin in most deployed software,
it's just an empty resource. So this MUST doesn't
make sense in the context given.
This advice could be conditioned on the HTML interpreter
actually being compliant with html5, but then this
doesn't really belong in the registration of about:,
whose use is already widely deployed.
...