There's a standard serialization of standardized DOM objects in which everything is a string. But the fact that the serialization is a string isn't very interesting, because pretty much everything has a serialization or easily could easily have a serialization where everything is a string; the underlying capabilities of what is being described is the interesting thing, and is where all the differences between data types and formats arise.
HTML is not a serialization of code objects. It is a document authoring format. The DOM is just a way of modeling the documents so that code can perform various transforms on them. But the document is the important part, not the model.