What are the differences and issues between NN and IE?
This question is normally asked in conjunction with the question: How do I get my page to look identical in both browsers. Apart from the obvious - there is no such thing as both browsers - since the user has a choice of close to 600 browsers.
Typically, this question comes from designers that haven't quite got the grasp of the essence of hypertext media and its related HTML markup. HTML, by design, is a structural document markup. Document structure, not layout. With that in mind, HTML is seen as not a high level precision layout language, although Cascading Stylesheets give the web designer the ability to precisely position objects, with certain limitations.
On the basic level, the difference between Netscape and Internet Explorer is that Internet Explorer is more forgiving of markup errors. This forgiveness is a double-edged sword, in that Internet Explorer will not render what you ask, but render what it guesses you want to see. Normally this works out pretty well, but on the wrong occassion it gets things completely wrong.
Authoring Internet Explorer only type websites is contradictory to the spirit of the World Wide Web - which assumes nothing about networks, operating systems, browsers and user agents. To rely on certain "usability" features of Internet Explorer restricts the usage of your material to that particular browser.
One of the main rules of good web design is to never assume what the user has, and not to rely on features of one browser for your presentation. Breaking these rules will only rebound on the webmaster, and will damage the web-site's reputation in terms of quality.
The only reasons to understand the differences between Netscape (4), and Internet Explorer is so that these differences can be avoided, rather than duplicating functionality for the sake of two browsers.