You define an internal schema, which is essentially just a tree structure according to your own use-case. In this way it lets you do editing of more complicated

You define an internal schema, which is essentially just a tree structure according to your own use-case. In this way it lets you do editing of more complicated page elements than a typical WYSWYG editor. You can provide (or use others') serialisers and deserialisers to convert between that representation and something else, e.g. RTF, HTML.