Javascript WYSIWYG editors do not use a textarea (at least not externally, it is likely that behind the scenes there is a textarea that is populated with the code that makes up the WYSIWYG content so that it can be posted in a form).
Rather, there are two properties that are used to make an editable area in a webpage: designMode, which applies to a whole document, or contentEditable, which applies to a specific element. Both properties were originally Microsoft innovations, but have been adopted by all major browsers (contentEditable is now part of HTML5).
Once a document (in terms of rich text editors this generally means embedding an iframe with designMode set into your page) or element is made editable, formatting is done by using the execCommand method (for which there are a number of different modes -- bold, italics, etc. -- which are not necessarily the same across all browsers. See this table for more info).
In order to pass content from the editable element to the server, generally the innerHTML of the editable element, is loaded into a textarea, which is posted. The makeup of the HTML generated depends on the browser.
Resources:
- http://blog.whatwg.org/the-road-to-html-5-contenteditable
- http://www.mozilla.org/editor/ie2midas.html
