WYSIWYG HTML Textareas

I've seen many comments about WYSIWYG editors, but I'm guessing they're
mostly about html page producing editors. What about WYSIWYG HTML
Textareas... do the same problems, and solutions, present themselves as they
do for WYSIWYG page editors? If we can expect a WYSIWYG page editor to do
semantics, can we expect a WYSIWYG HTML Textarea to do the same? What if the
application (blog/forum/wiki/whatever) doesn't support everything?