Here's a bug I've noticed a little bit ago, not sure if this is is persona related or Nbb, but if you make the window smaller (heightwise), the composer will begin to get cut off and you can't see what you're typing.
Hi, I'm the s9e\TextFormatter author. I have a couple of Google Alerts so unlike Beetlejuice you only have to save my name once for me to appear. 🙂
The client-side rendering is performed with XSLTProcessor. Adding an MSXML fallback for IE is easy. Rendering in pure JS is technically feasible (there's already a pure PHP renderer) but I never got to it because I have no use for it. The library could be used as the base for a true WYSIWYG editor (where what you see is actually what you get because there's no conversion from HTML back to Markdown or things like that) but that would be a big project. I'd be willing to discuss any type of collaboration if someone's interested.