WYSIWYG or What you see is what you get - it sure sounds good. Visual designers love the idea of a tool that allows you to manipulate the elements of a Web page by just dragging them around on the page and placing them exactly where you want them. In fact, there are editors which do exactly this - and don't even let you look at the HTML code. But I'm not convinced that they actually are "what you see is what you get". Instead, they are "what you see is close to what you get in the right browser".
This Is Not a Rant Against WYSIWYG Editors
WYSIWYG editors are a useful tool. The allow visual designers the chance to see what they're building without having to take the extra step of loading up the page in a browser. They make things like tables much faster to build because you can simply draw what you want your table to look like, and poof it's there.
Browser Testing is Still Required
WYSIWYG is nominally true if you are the only one viewing your Web page. But creating a Web page in a WYSIWYG editor does not relieve you of your duty to test in multiple browsers and operating systems if you're planning on showing it to anyone else.
One of the most frustrating situations I had was building a page in Dreamweaver. Now this is arguably the best WYSIWYG editor available, but when I built it in WYSIWYG mode the page looked great. Everything lined up perfectly, and was exactly what I wanted. Then when I uploaded the page and tested it in IE and Firefox, it blew up. The layout was messed up with things overlapping and not floating correctly. It was a nightmare.
What was especially interesting is that Dreamweaver allows you to view the pages in the Web browsers as well as in the WYSIWYG viewer. I did that and they looked great there too. The problem happened when the page was uploaded to the server. But it's not just the server that can affect your pages.
Just because it looks great in your WYSIWYG editor doesn't mean it will look great when it's viewed in another browser. Designers that use WYSIWYG editors on Windows often have problems when their pages are viewed on a Macintosh or Linux system.
WYSIWYG Editors Are a Tool
Use WYSIWYG editors as a tool, but not as your only judge of whether a design works. The Web is not print, so we can't ever guarantee that a page will look identical to your expectations. As long as you know that and test for that, you should be fine using a WYSIWYG editor.