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.
s9e\TextFormatter is a PHP library with a JavaScript port. It uses plugins to let the user pick and choose what kind of features they want. The JavaScript version is generated by the PHP library, which means you'd need PHP as part of your build process. Once the JavaScript is generated it can be redistributed easily so I guess you could bundle a preconfigured/pregenerated version. You can see a couple of live demos there: BBCodes+stuff demo, Fatdown (Markdownish+stuff) demo.
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.
