The anonymous donor was Aha! (http://www.aha.io). It wasn't as anonymous as we expected because Marijn tweeted it :-), but we are excited by ProseMirror and the opportunity to contribute. WYSIWYG editing is a huge part of what our app does, and so I have always been on the lookout for better ways to build editors. Marijn's approach of using a shadow document representation (http://marijnhaverbeke.nl/blog/prosemirror.html) is the most promising solution I have seen for a while.