A lot of people must have been holding out on me, because this is the first I’ve heard of editing platforms built into Internet Explorer (MSHTML Editing Platform) and Mozilla (Midas). Look, I may have slept with your sister, but there was no need to hold something like this from me 
Here’s a “Cross-Browser Rich Text Editor” that uses these techniques: Cross-Browser Rich Text Editor
Something tells me Safari hasn’t bothered to implement either, right? 
You asked for it punk. From Dave Hyatt himself (look it up):
“HTML Editing
Safari 1.3 supports HTML editing, both at the Objective-C WebKit API level and using contenteditable and designMode in a Web page. The new Mail app in Tiger uses WebKit for message composition. You can write apps that make use of WebKit’s editing technology and deploy them on Panther and Tiger.”
We’re livin’ in 2005, that’s old news:-) Hence the fundamentalists are gone that praised pure HTML and JavaScript became somehow hip, everything is possible. There’s a bunch of Rich Text Editors, free and commercial, most of them rely on either one of the two kits. But there is much more possible than what we currently see, I can imagine some rapid application development tool, like Visual Basic or Delphi (or Glade, KDevelope and so on that came years later) which run completely thru your browser. It’s not an accident that the Googly (which is hyped far too much) guys hired those Mozilla hackers. Ah, this is not meant as critics, I’m sure your dom/js knowledge is much bigger than mine, I’m just amazed that you didn’t heard of this before. I hope you didn’t sleep with my sister…