Can Macro Return Something if Triggered by Web Trigger?

Can a macro return some values when triggered by a web trigger? It would be very useful to be able to return something like json for the client that triggered the macro to parse the result.

No, there is no current mechanism for sending information back (it is not practical for a number of reasons, not least being that it is somewhat asynchronous, and also that you can trigger a macro on multiple different Macs with a single trigger so there is potentially multiple results.

Your macro can potentially make information available via some other mechanism (such as uploading information somewhere or trigger a reply macro via a different remote trigger on the source Mac).