While this format is more generic, an abbreviated encoding can sometimes accomplish the same thing. For example, just moving the "type" to be the object key and removing the implied secondary name gets you this far:

    { "mrow": [
        { "mi": "x" },
        { "mo": "=" },
        { "mfrac": [
            { "mrow": [
                { "mo": "-" },
                { "mi": "b" },
                { "mo": "±" },
                { "sqrt": {
                    { "mrow": [
                        {"mi": "b"},
                        {"msup": { "mi": 2 }},
                        {"mo": "-"},
                        {"mi", "4ac"}
                    ]}
                }}
            },
            { "mrow": [
                {"mi": "2a"}
            ]}
        ]}
    }

It's not as general, but works if you know your syntax is similarly bounded. I don't know how certain static languages would handle serial/deserializing, but makes construction via javascript literals much more pleasant.