My response to the first two questions in this SitePoint thread:
Learn XHTML “STRICT” and get yourself a XHTML Quickstart guide to browse.
When you’re first starting out, run your code through an XHTML validator to see your problems areas. Don’t get carried away with STRICT, think of it as an ideal. At some point you’ll realize you can’t please the validator 100% of the time and you’ll start to worry more about how the page displays in Firefox and MSIE.
Google admits it does not care if you are STRICT because most of the code on the web doesn’t validate anyway. Sometimes Firefox is more forgiving of errors, sometimes MSIE is more forgiving of errors. In general, I find IE a little more strict than Firefox, assuming you’re not in quirks mode.
It comes down to trial and error testing in Firefox and MSIE, there will always be some unique situation you haven’t encountered before. I recommend you get yourself very familiar with Firefox, don’t do anything too tricky with your code, be very specific about everything, and you will find the page renders about the same in IE most of the time with few modifications.
Discrepancies are usually due to lack of specificity, you leave these browsers up to their own devices (default behavior) and they will surprise you.
I recommend you ignore all the idealism about various measurement systems and stick to pixel measurements for everything. Most users don’t have gigantic screens and will notice if an element is just one pixel out of place. When you specify pixel measurements you will never be surprised. Anyone with a freakishly large screen is going to be disappointed by everyone, not just you, and Ctrl+ solves that problem.
I set fonts, margins, borders and padding in “px”. Master how those four work and you can solve just about every design problem you encounter with CSS. Many designers will disagree with me for saying this, but don’t be afraid of using inline “style=” tags, inline CSS has its place and will save you a lot of time and bandwidth. A large CSS file can slow down your entire site and hurt your UX rating in the search engines.