Cardinality is the easy way to resolve this. If the data has a cardinality of 1, it should be an attribute. If cardinality > 1, it should be a child element/node.
It’s not easy though. When we started with html we assumed images could only have one source. It was obvious that cardinality was 1. Later we realised we could have multiple versions of the same image. With audio tags that was resolved.
I’ve come to realise that it’s almost always a mistake to assume a cardinality of 1, unless it’s something that is defined to be artificially unique like the id attribute.
I’ve come to realise that it’s almost always a mistake to assume a cardinality of 1, unless it’s something that is defined to be artificially unique like the id attribute.