wikipedia.org: https://en.wikipedia.org/wiki/DMARC: The content of the TXT resource record consists of name=value tags, separated by semicolons, similar to SPF and DKIM. For example:
“v=DMARC1;p=none;sp=quarantine;pct=100;rua=mailto:dmarcreports@example.com;”

Inspection, eg with: https://dmarcian.com/dmarc-inspector

Points to watch out for:

1. Reporting to an email address elsewhere requires authorization in that external DNS.
2. Choosing ‘s(trict)’ instead of default ‘r(elaxed)’, a subdomain also needs authorization.
3. A DNS value for another domain ‘v=DMARC1;’ requires a semicolon (the original example was wrong).
4. Extra spaces destabilize. The testing tools do respond differently.
5. Lines with ABNF syntax, as for DMARC, can optionally terminate with a semicolon.
6. The file size can be limited to, for example, 10 megabytes with !10m at the end of the email address.
7. Eg _dmarc and _dmarc.www can derive using ‘CNAME _dmarc.reject.janwillemstegink.nl.’ set up with:
   ‘TXT v=DMARC1; p=reject; sp=reject; np=reject; ruf=mailto:admindesk@janwillemstegink.nl!10m;’

https://tools.ietf.org/html/rfc7489#section-7.1 (Request for Comments for DMARC):

For example, if a DMARC policy query for “blue.example.com” contained “rua=mailto:reports@red.example.net”, the host extracted from the latter (“red.example.net”) does not match “blue.example.com”, so this procedure is enacted. A TXT query for “blue.example.com._report._dmarc.red.example.net” is issued. If a single reply comes back containing a tag of “v=DMARC1”, then the relationship between the two is confirmed.

https://tools.ietf.org/html/rfc6376#section-3.2 (Request for Comments for DKIM):

Formally, the ABNF syntax rules are as follows:

tag-list = tag-spec *( “;” tag-spec ) [ “;” ]
tag-spec = [FWS] tag-name [FWS] “=” [FWS] tag-value [FWS]
tag-name = ALPHA *ALNUMPUNC
tag-value = [ tval ( 1(WSP / FWS) tval ) ]
; Prohibits WSP and FWS at beginning and end
tval = 1*VALCHAR
VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / “_”

Note that WSP is allowed anywhere around tags. In particular, any
WSP after the “=” and any WSP before the terminating “;” is not part
of the value; however, WSP inside the value is significant.