I've done a ton of work with contenteditable and it's not especially good on IE (it was especially bad on IE up to IE8) nor especially bad on Safari (it's kind of horrible, but it's horrible everywhere).
I edited my text, I meant: Safari on iOS. The touch interface introduces bugs that haven't been fixed for years. Desktop Safari is as buggy as Chrome, both originally based on the same Webkit code.
GoogleDocs v2, Word WebApp and iCloud Pages use a JavaScript based page layout engine that renders one DIV per line and don't use the contenteditable. Even the blinking cursor is a DIV. Though, even they do not support text-columns - something that would work out of the box with contenteditable. (GoogleDocs v1 used contenteditable, as well as SharePoint content editor and its first Word WebApp incarnation)
Implementing such a complex page layout engine plus internal document format (XML based?) is a lot of work and afaik there is no such open source JS library exists.
Agreed. For instance, IE (11 at least) doesn't permit <td> cells contenteditable. There are some workarounds, but then other browsers seem to have problems.
