You can merge successive elements, even if they are not at the same level in the document structure, by using the Backspace and Delete keys. When the insertion point is at the end of an element, use the Delete key to merge; when it is at the beginning of an element, use the Backspace key. In both cases, these keys do not delete any characters; instead they merge elements previously separated by a structure boundary.
Note that when the insertion point is within an element, the Backspace and Delete keys delete the next or previous character as usual.