Years ago, I “learned” that made-up attributes (e.g. node [label="OK" doghouse=5]) do not cause error or warning messages, but instead are attached to the appropriate object(s) and passed through to any interested user downstream (useless if output format is png but possibly useful if format is svg, dot or json).
Is there any documentation for the feature?
You have documented it! That’s about it! For security reasons we have really locked down the values passed through in SVG. Not sure about json. It seems “easy” [sic].
This is documented here for the id attribute. I can’t find any such documentation for attributes in general.
Generally you ignore the unknown fields otherwise new graphs will error on old graphviz. Makes the software more robust.