The term 'mixin' is colliding with what an OOP mixin is. Its use also is not entirely correct – for example a MapNode is not visible in XML, but it is visible in JSON.

Determine whether this marker is useful at all and if it is, rename it to a better name and perhaps split it for XML/JSON cases, or find a general language around it.

My preference is removing it, as it brings in codec knowledge, and there are more than two of those.