The PNG Metadata Format Specification in the package description for
javax.imageio.metadata has the text
"The semantics of the standard chunks are described in the PNG specification"
which includes a hyperlink to http://www.freesoftware.com/pub/png/spec/
Unfortunately that domainname doesn't seem to be valid any more and it takes you
to a site full of commercials.

We should instead link to http://www.libpng.org/pub/png/spec/
as is done in the table in the package description for javax.imageio