Timelines are often used in novel writing. For example, the timeline workflow in Manuskcript. The concept can serve a similar purpose in writing historical articles but would probably hinder writing math proofs or technical manuals. For an editor, the feature should be accessible but hidden by default.
CSS is amazing, but the HTML DOM isn't. It's just so heavy. Maybe if JSON or some similar format could replace HTML/XML, it could be made lighter? Better, I think, would be to use a human readable format like Eno . It's as terse but much easier to read than JSON.