As a side comment: HTMLmirror vs ProseMirror:
Being an editor for HTML pages, I think what end users and developers want is a HTMLmirror editor instead of a ProseMirror editor (that focuses just on prose).
That is, a HTML editor that gives control over HTML content in a WYSIWYG way and can also handle pasting of complex HTML content (retaining the fidelity of the original HTML and then be able to edit it). That’s a lot to ask for … but that’s what users really want for HTML pages…
I just give this opinion to give input on your future direction for the project and especially the future scope of the project (prose vs more expansive HTML). ProseMirror is a great contribution and seems to be by far the leading editor for mobile browsers. Mobile is dominating the web, so ProseMirror is well positioned to be the basis of a powerhouse on the web…