What You See Isn't Always What You Get.
When you use a visual HTML editor like FrontPage or Dreamweaver, they tell you that WYSIWYG: what you see is what you get. This means that, in theory, what you see while you're editing the page should look just like what you get when you're done. While word processors have pretty much got the concept working now when it comes to printing (it didn't used to be as reliable), there are still problems when it comes to HTML editors and web browsers – of course, if you've spent more than five minutes using visual HTML editors, then you'll know that.
The Advantages of WYSIWYG.
WYSIWYG is quite uncontroversial at this point – its advantages are clear, in that it lets you make your page look exactly the way you want it to while you're writing it, with no surprises. In the early days, it was sometimes referred to as being like a kind of 'interactive print preview'. Professional typesetters were at first opposed to its use in publishing, but it's easy and quick enough that it has come to dominate. The alternative now, though, is to go back to the days of layout commands, which isn't something most users want to do.
On the web, WYSIWY it can be far more problematic, because what comes out of the other end of the program isn't a static, printed page – it's computer code, HTML, that has to be interpreted by a web browser before it becomes anything viewable.
Not Even All Browsers are the Same.
The first problem with what you see in an editor being what you get in a browser is that all the different browsers available don't always make the same page look the same way. How are HTML editors supposed to account for bugs in Internet Explorer? They can't, really.
Each piece of HTML editing software is forced to either write its own HTML rendering engine (the engine that decides how the code is translated to a visible page), or use one from an existing program. Recently, for example, Dreamweaver moved over to Opera's engine, which means that it shows pages the way Opera does. FrontPage has always been closest to Internet Explorer. Because Mozilla is open source, there are a lot of HTML editors based on its engine, the most usable of which is Nvu.
That doesn't exactly help, though, when it comes to things looking the same in every browser – if you use Dreamweaver, for example, what you see will be what you get in Opera, but not necessarily in Internet Explorer. This is a problem that can be partially solved by testing everything in every browser, but doing that doesn't let you see what your page is going to look like as you're going along.
Maybe What You See Shouldn't Be What You Get.
While users demand WYSIWYG software, it's somewhat misguided when it comes to the web, for the simple reason that it expects everyone to be using your site the same way, and designs towards that expected use. In reality, the web was designed to be a document format that was interpreted by the program receiving it, meaning that if a browser wants to leave out all the graphics, or ignore all your tables, then it's perfectly justified in doing so. This is especially significant when it comes to mobile browsers – they simply don't have large enough screens to display normal designs, and it's silly to force them to try.
Realising this is one of the most important differences between being a good designer and being a bad one. Bad designers will be constantly nudging at their designs, doing everything they can think of to get them to look exactly the way they intended in every browser possible, even if it doubles the size and complexity of the code. A good designer will write good code that displays in all browsers, but doesn't necessarily have to be pixel-perfect.