It's impossible to have a WYSIWYG editor in a wiki without using some additional technology, like javascript, an applet, or a browser extension. But it's possible to have "almost wysiwyg" by carefully choosing the markup in the RawText, so that it is similar to the RenderedPage.
The most widespread example of MostlyWysiwyg is using the CamelCase to mark links in wikis. Other examples might be using "_" for _underline_, indentation for blockqutes, indented single asterisks for bullet lists and a row of dashes for a horizontal line.
MostlyWysiwyg has many advantages of WYSIWYG, but without the technical limitations of the latter.
On the other hand, MostlyWysiwyg as a principle breaks down when users wish to enter tables, in-line images, and links other than CamelCase. If what you want to get is a link to the Indiana University football program with the text "IU Football," you have to type something like this into the editor:
"(IU Football|http://iuhoosiers.cstv.com/sports/m-footbl/ind-m-footbl-body.html
)."
This can hardly be called seeing what you'll get.