First, I'm a noob. Don't do this professionally - only as a hobby.
Second, I've gotten some wonderfully generous help on this forum and hope some day to be able to pay it forward.l
Now, I've built a small site with a very limited set of users with scant traffic. I'm trying to adopt best practices and avoid deprecated elements, etc. I write directly in an editor, not a generator. I'm a rank beginner, but I've learned some HTML, PHP, CSS. My pages work very nearly as intended in Fx, but IE is driving me nuts. Op displays my intentions better than IE but not as well as Fx.
My question for you is, "What tools, techniques, websites, books should I be using to learn how to find the deltas and address them?"