That was easy
Well things are about as fixed as they are going to get. For those wanna-be web designers out there, layers give you temendous flexibility and you can do some amazing things with the, but they are a serious pain when trying to get them to look the same in Internet Explorer & Mozilla/Netscape. They render differently and you'll get a nice "close but no cigar" situation.
Mix in CSS and you're asking for more trouble. It can be done but you have to test and plan ahead and don't let the browsers work around your code. You have to make sure everything that is supposed to be marked up IS. One simple change in my CSS fixed a major issue with my page. The different browsers will simply treat the code different. Leave something out on one and it works, but it'll mess up the other.
Another unfortunate gotcha is using font size. Font size 2 will give you about 12 pixels in IE but only 9 pixels in Mozilla/Netscape. Who's right? Actually they both are. So you're better off using pixel heights when defineing font size. The browers have to display this size the same.
The bottom-line is, when developing a page, start simple and test often with all the browsers you feel should be tested against. I recommend Internet Explorer and either Mozilla or Netscape 6/7. Mozilla and Netscape use the same rendering engine, so they should, in theory, be identical. Keep adding and testing, adding and testing. When you run across a trouble maker, play with your options until you fix the problem or consider another solution.
Sorry to those whom I have gone over their heads. Don't sweat it. If you aren't a developer you won't need to worry about this stuff anyway.
To my fellow bloggers, Sugarmama is the only one who is having Mozilla problems.