Some early rich text editors never used a tree representation for formatting represented the formatting as "control character sequences". This was part of why some people loved WordPerfect so much because you could toggle a view of all the control characters and just delete/copy/paste them like any other text in the same document.

It's basically how the classic RTF format [1] works, and things like VT100/ANSI escape codes in terminals. It's kind of like the difference between imperative code and declarative code: "this character sequence means toggle the state of bold" versus "this node of characters is bold".

[1] https://docs.fileformat.com/word-processing/rtf/