In the beginning Tim Berners-Lee made his original browser double as an WYSIWYG editor. Sadly this was not to last. Since then we have had two types of editors:

1. desktop application like Dreamweaver, Homesite, etc.
2. Java applets or Javascript running on browsers.

Now there is a truly browser neutral WYSIWYG HTML editor thanks to Stuart Schoneveld - another example of Flash's power of delivery and deployment. Given that Flash can be packaged into an executable, someday soon we may get an usable editor on both desktops and webtops, cross platform.

References: Timothy Appnel's blog Jon Udell's blog and his column The Universal Canvas