Change my view: given any data storage medium, the smallest granularity of data must also be the most-child element of any markup language. Given the immense overhead of storing markups on a granular level, processing markup therefore must be a perpetual exercise in recursion.

I.e.

      Poem->Verse->Line-> <char>

      Book->Page->Chapter->Paragraph->Sentence->Word-> <char>

      HTML->Body->Div->P-> <char>

Therefore, any given letter (here as a <char> type) can retain a back reference of parents, so the <char> object retains a hashset of {Line,Word,P} parent type references representing three domains, but really needs to be a Dictionary of key values, the key being the domain name, the value being the parent name, so that would be:

Domain: Poetry, Value: Line

Domain: Book Object Model, Value: Word

Domain: HTML, Value: P Element

We could then ask any letter arbitrarily "what is your Font Style in your HTML context?" and it would be able to walk up the parent P which obtains its style from a CSS markup, and return that correctly. Or "What is your Poem's name in your Poetry context?" and it could recurse up to the Poem element to find it's Title.