I stumbled upon a slight difference in zonfile validation with nsd-checkzone compared to the named-checkzone from bind9.

Assuming this TXT record in a zonfile:

foo       IN  TXT   "a"b

Please note, that there is no whitespace between "a" and b.

• named-checkzone considers a zonefile with such a TXT record valid. If loaded into bind, the zonefile is accepted and responds for the record.
• nsd-checkzone exits with a syntax error, having such a record in a zonefile also prevents NSD from starting. Only if you add a withespace between "a" and b, nsd-checkzone accepts this.
• kzonecheck (knot dns server) has the same behaviour as nsd on this topic.

I could not find the section in any RFC that covers this, but I think zonefiles should be cross compatible with bind providing the reference.