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.