Of of the top of my head, you'll need to watch out for:

- The default behavior that triple-click to selects a whole paragraph/line will stop at the first contenteditable=false element.

- Android may dismiss the keyboard & selection if the selection ever touches one of your contenteditable=false marker elements.

- Firefox won't let you place the caret in between two contenteditable=false elements, or at the start or end of a contenteditable=true element if the first/last child is contenteditable=false.

Why not use ProseMirror?