I don't think you are quite getting it yet. The user, meaning an author, who is working on a structured document, should simply insert a new <b> element and type whatever text is supposed to go into that <b> element. If a piece of text is already there, the author selects the text, selects the <b> element in the element catalog and wraps the text into the <b> element, instead of inserting an empty <b> element and then start typing. There is absolutely no need for the functionality you are asking for, as all of this is already available.
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