如何制作HTML/JS WYSIWYG编辑器?

I've tried many different Google searches but I haven't been able to find a current tutorial (newer than 2006) on how to actually create a WYSIWYG editor. I realize there are many already, but I'm curious as to how they actually work. I've looked over some of the source code, but it's a lot to digest. It seems formatted text can't be placed in a textarea box, and yet they give the illusion of doing just that - how? I've tried many different Google searches but I