Don't expect the browser vendors to fix contentEditable or inconsistencies for you, this has been the struggle of WYSIWYG editors for as long as they have existed and it really hasn't gotten a whole lot better. You will quickly find out that by the time your getting close to fix those inconsistencies, 4-6 years have passed and your right up there with the rest of the editors in terms of size and complexity.
PS: Also working on one of those bloaty editors.