There might be a quibble about whether we want separate open/standalone/close productions or separate / from } on standalone, but I think this is pretty darned clean.

ACK.

I thought about this, and decided to use this style to bring it closer to the json data model, where we have 3 completely separated structures for markup-open/standalone/close.

They are also not unified in the data-model/README:

type Markup = MarkupOpen | MarkupStandalone | MarkupClose;
interface MarkupOpen {
    type: "markup";
    kind: "open";
    ...
}
interface MarkupStandalone {
    type: "markup";
    kind: "standalone";
    ...
}
interface MarkupClose {
    type: "markup";
     kind: "close";
    ....
}

On the other side, in the data-model dtd they are all unified.

I would prefer the unified style.

So the readme would be more like

type Markup = MarkupOpen | MarkupStandalone | MarkupClose;
interface Markup {
    type: "markup";
    kind; // one of "open", "close", "standalone"
    ...
}

But I didn't want to include anything controversial.

I would also like to fix this issue: "Extra spaces in markup #650" (that I've just created)

All feedback implemented, thank you all!
(ready for round two :-)