It is very painful to develope a KILLER HTML layout only to discover that it doesn’t work well at all in browser X. Do yourself a favor and test it as you go. After you’ve finished creating a new phase in your layout, test it in several other browsers. Older Netscape versions tend to do very odd things to seemingly standards compliant code. I personally use Opera for most of my development and then test in IE, NS 6, Moz and finally Lynx to see what the spiders will actually see. Yes, Lynx is NOT graphical, but it will give a very clear picture of the way things are parsed by the spiders.
Part of testing should include Syntax Checking. Many checkers are available. If you don’t have a program that has a checker, you can use this service from the W3C site http://validator.w3.org/. (Opens in new window) As I note elsewhere, not every piece of code that I do will not pass strict checking. To me, there are certain things that I don’t mind. Strict HTML checking will throw errors when a flag is not accepted by all browsers equally. Several IE-centric items will throw an error under strict HTML checking, but if you know what your desired effect and result should be, don’t sweat it. But don’t ignore it either!