GNOME/Evolution bug https://bugzilla.gnome.org/show_bug.cgi?id=752166 covers a WYSIWYG failure, where (plain text) messages edited with WebKit are not sent in the form which is actually being displayed to the user at the moment they hit 'send'. It is alleged there that WebKit does not provide a way for Evolution to *know* how the text was being rendered on-screen, and thus that getting WYSIWYG right is fundamentally impossible. If true, this would be quite a serious lack. It basically makes WebKit entirely unsuited for this class of use cases. Obviously, it's entirely unacceptable for us to have to have an external renderer which *approximates* what we think the current version of WebKit might do today. Especially given that we don't *see* the output in most cases, unless we go looking in our Sent folder to see what actually got sent. (I actually did that, just now. It made me very sad.)
I don't know what we can do with this bug. If you could suggest which API are missing, or which features need to be created, we could take action. For now, this doesn't seem like something we can take action on.