epubcheck doesn't recognize reserved a11y namespace

[acabal]

epubcheck version v4.0.2
Ubuntu 16.04

We're trying to implement the epub a11y spec in our ebooks, where in section 1.2 it states that:

The prefix "a11y:" is reserved for use with properties in this vocabulary and does not have to be declared in the Package Document.

However if you run epubcheck on an epub which uses the a11y prefix, it outputs this error:

ERROR(OPF-028): /tmp/my-ebook.epub/epub/content.opf(22,59): Undeclared prefix: 'a11y'.

This can be worked around by declaring the a11y namespace in the package OPF, but according to the spec we shouldn't have to.

It's unclear to me if the a11y spec depends on epub 3.1 (which I know is not supported by epubcheck at the moment) but in case it isn't, I thought I'd open this issue.

[acabal]

D'oh, this is #810. Closed as a duplicate, sorry!