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I don't think there's any WYSIWYG editor out there that will display exactly what the end product will be. FP has the Preview mode for a reason.
One thing you have to be wary of is using FP as a CSS editor/WYSIWYG. CSS is a fairly recent addition to FP, and it's not the best implementation. I've used TopStyle as my CSS editor for several years now. It plugs directly into FP and works nicely in that capacity.
I don't know what sort of experience you have designing websites, but be aware that different browsers will display your site differently. It's not the best idea to try to micro-manage site design for that reason. You can't expect a website to look the same on every operating system, screen resolution, or browser. Whatever FP shows you in WYSIWYG mode is going to an approximation at best, but it's an approximation that helps you get the basics done quickly without the need to hand-code everything.
You may be expecting a bit too much of the software. A WYSIWYG editor is just one of many tools you'll need to make a professional site. Unfortunately there isn't one software package out there that does it all.
You can't expect a website to look the same on every operating system, screen resolution, or browser.
Exactly. Contrary to what a lot of print-trained graphic designers might hope or think, Web design isn't like desktop publishing. If it were, browsers would read PostScript, not HTML...and pages would take half an hour to display at dial-up speeds. :-)