> And no similar open source project exists.
Check out http://quilljs.com -- it's a cross browser rich text editor that does provide such an api.
Granted, it doesn't yet have all the functionality of a complete word processor, but it's early days and its module system provides good extensibility. Part of what's exciting to me about it is that it's also a good chance to rethink what a word processor should provide in the context of the web, instead of simply porting what made sense when the target was printing a document to paper.
We need better WYSIWYG-API implementations for HTML5. Sure, it would mean competition to web office apps from Microsoft, Apple and Google. A Word-clone web app could be a single HTML5 code line, but at the moment due to the broken API e.g. Google Docs renders the page line by line as seperate divs, even the blinking cursur is a div - it's a complex full page render engine based on divs!! And no similar open source project exists.