I think unicode, fonts, etc would be an issue even with a plain text editor anyway.
The "styling a range of text" is something i thought but you still need to somehow associate the text with the range - and vice versa - and this doesn't handle things like inserting images and other types of objects since these aren't text.
You could have a document be a series of "paragraphs", each being a series of "elements" with each "element" being something like "text" (with a style), "image", etc. But then once tables enter the picture, you need to expand paragraphs to be of "table" type and each table cell is itself a self-contained "series of paragraphs" - and then start thinking about nested tables or images in tables!
Generalize that enough to avoid special cases inside special cases and you end up with more of a tree-like structure representing a DOM and less with a linear structure with range-based styling.
(of course, then again, i don't remember Write for Windows 3.1 having tables in the first place :-P but i'm interested if there are alternative approaches anyway)
EDIT: one thing i forgot to mention - and why i am curious about non-DOM-based approaches - is that one problem with the DOM approach is the selection: with a linear/range-based structure the selection is just one or two indices inside the range, but with the DOM the selection can start from a node with node-specific subrange (e.g. character in a text node) and end with another node and both being very unrelated to each other (i.e. only having some distant common ancestor and not necessarily at the same level).
