I should also point out that NN4.x is very unforgiving when it comes to improperly nested tags. If there is one thing out of order, or a closing tag missing, it chokes and renders your CSS improperly.
Here are some of the problems I found and fixed while making my sites cross browser friendly.
1. <form> tags work better when they are enclosed in a block element like a <p> tag. If you don't have a block element surrounding the <form> tag, NN4.x does not render any CSS after it!
2. I work in FP. Every now and then FP has a bad habit of inserting <span> tags when I make a change to something. Its not real frequent, but when that <span> is there, NN4.x chokes on the CSS and does not render anything after the <span> correctly.
3. NN4.x does not like <p align> tags. I've found that if you are using tables, you should specify the alignment in the cell and not the paragraph. The <p align> works but not in all instances, depends on where it is and what surrounds it.
4. NN4.x does not like <h> tags without a font-family assigned to them. You won't catch this unless you have your default font for browsing set to Times. I have mine set at Arial because I hate times and every now and then I'll catch a block level element that needs to have a font-family assigned. You would think that assigning the font family to the <body> would do it, but it does not. I believe the same issue applies to <li>, <ol> and <ul>.
Those are just some of the easy ones that I've encountered. I'm fortunate that the CSS I've been working with is compatible in all browsers, I made sure of that. No float:right or float:left, I've been sticking with non-fluid layouts. Once NN4.x falls off the face of this earth, then I'll play with those properties and values. Until then, I'm limited in what I can do design wise.