Hi Luigi,
I did a test with mkiv (see the tex file attached), and then tested the resulting pdf file on the site you mentioned,
http://www.pdf-tools.com/pdf/validate-pdfa-online.aspx
Unfortunately it says that the PDF produced by mkiv is not conform:
test-PDF-A-1b.pdf does not conform to PDF/A.
Validating file "test-PDF-A-1b.pdf" for conformance level pdfa-1a
Invalid content of XMP packet header attribute 'id': 'dtlnfbrryjnvabnrajbkrpyw'.
dc:description :: Wrong value type. Expected type 'lang alt'.
The XMP property 'pdfaid:conformance' has the invalid value 'B'. Required is 'A'.
The key MarkInfo is required but missing.
A device-specific color space (DeviceGray) without an appropriate output intent is used.
The document does not conform to the requested standard.
The document contains device-specific color spaces.
The document's meta data is either missing or inconsistent or corrupt.
The document doesn't provide appropriate logical structure information.
Done.
So, I don’t know how to solve the issue of creating a PDF/A.