The Problem why they did not implement this that you can't go back to Delta's. Quill works with Delta's. From those Delta's you can extract Html but from that Html you can't go back to Delta's. This is how Quill works.
Example: I have a tag <h1>TEST IF <span style='background:yellow'>HIGHLIGHT</span> WORKS</h1>
While editing the delta's for this code are generated by the Editor but if you edit this Html part it can't convert back to Delta's.
Why it works this way? The editor makes the Delta's when you selected a part of text where the effect must be created. Thats when the Delta is created and thats how Quill knows how to create Html.
Missing in Quill: A HtmlToDeltasParser
that creates Delta's from HTML and after
that put those Delta's in the Editor.