> The rest is just the UI

"just contenteditable" is really understating things. contenteditable is a god awful API full of bugs and inconsistencies. Making something reliable on top of it is a very significant amount of work.

For context: my day job for last 2 years has been building a new browser engine from scratch, and I think the contenteditable-based WYSIWYG editor I wrote 15 years ago might have been a harder project (albeit I was lot of less experienced then).