To let TinyMCE become more WYSIWYG the tinymce_plugin should use/setup the TinyMCE content_css setting.
This would add those CSS files to the TinyMCE instance and as a consequence the HTML in TinyMCE would look as similar as possible to the final result.
To accomplish this we have either to autodetect the blog skin's .css file(s) (e.g. by looking at the Blog's skin, retrieving the baseurl and then parsing out any .css links) and/or allow the admin to configure this.
See http://wiki.moxiecode.com/index.php/TinyMCE:Configuration/content_css