execCommand was used to add rich text to web mail clients, it's fine for writing once but not exchanging documents between browsers because of different interpretations at to what the commands do and how they handle nested tags ala <i><b></b></i> or <b><i></i></b>. Anyway prosemirror has a schema that provides extra info to how the tags should be handled: https://prosemirror.net/docs/guide/#doc Also this is a much bigger design area then you might think.