Just a note on my thoughts at the moment:

We can probably achieve this in a similar way to the RNG, where we have a separate class that you can call methods on: https://dice-roller.github.io/documentation/guide/customisation.html#random-number-generator

So we'd have a class (e.g. Output that you can pass an engine or similar names object to, that handles the output itself.

The engine will have to have a public method (e.g. output()) that takes the object to output (and probably a context object, which could contain things like the parent, for contextual output / styling):

type ObjectTypes = RollResult | ResultGroup ...;

interface engine {
    output<ReturnType>: (arg: ObjectTypes, context: { parent?: ObjectTypes | undefined }) => <ReturnType>
}

• Each object that can be output will need an output() method. When called, it calls a method on the Output class which in turn calls the output() method on the current output engine - passing in the object to output.
• The engine then handles the output in whichever way is appropriate and returns the response however it likes.
• This needs to handle opening / closing of output so we can do things like wrapping children in the HTML of a parent element. This might require a before() and after() method, or maybe wrap() which returns an array of two parts - start and end?
• A default engine should be created that simply calls the toString() method on the object.