The first thing I would like to say is, know your visitors. If your site is anything like w3schools and has more visitors coming with Firefox than with IE6 (though IE altogether has more)
http://www.w3schools.com/browsers/browsers_stats.asp (for those who don't believe) you will want to start thinking about making it accessible to all users. This means that since a third of your users are in FF you can't make the mistakes that lead to flash overlaying a drop down menu that drops down when you hover etc. You should also start being friendly to bigger resolutions in optimization, 1024x768 I feel is what you should set as standard. I'm willing to bet that most people have it and very few have 800x680 set. I'm also sure more and more people are using larger resolutions, 1280x1024 and higher, you want to make sure the site looks good to them, if you set everything to sit on the left side of the site and their screen is so wide that 1/3 or more of the page is blank on the right side you'll start losing people. You may also lose people who want to look at the middle of their screen, not the left side. I use
http://browsershots.org to check on how it looks in other browsers. I suggest testing in IE6, IE7, FF 2 and 1.5, Safari 2 and 3 (and 3 on PC) and Opera. IE6 and IE7 both have unique bugs in rendering, but they both use the Mozilla engine (not the Mozilla browser engine, which is Gecko, do some history research) FF 1.5 and 2 both also render a few things differently because they use a different version of Gecko (Netscape, Seamonkey, Thunderbird, the Mozilla Suite, Epiphany, Galeon, and the most recent version of Dillo {which hasn't been updated since '06 but otherwise would have its own engine from their homepage}) Safari uses the KHTML (they renamed it to webkit or something like that, but it's KHTML) which is also the engine found in Konqueror and was developed open source by the KDE team. I am willing to bet the Safari versions render things differently. Especially between mac and PC. Netfront I was unable to find out anything about from their site. Opera though, they are the lovely people who created their own browser from the engine on up, so that you can spend even more time trying to write a site to work in yet another display. So basically, 4 engines, Mozilla (IE only anymore, that I know of), Gecko (Netscape, Mozilla, Firefox, SeaMonkey, Galeon, Epiphany), KHTML (Konqueror and Safari), and Opera. Anyone who said, what about AOL browser gets two lashes of the whip. The first one for, AOL browser is just IE (as is compuserve's and all the rest of them), it's a skin. Secondly, AOL browser must die.
Now onto something beyond bending yourself to statistics. These are a few things that are actually useful to people and they may not have thought of them. The first is, if I was blind and using a screen reader, how would your site "look" to me? What if I was using a text-braille reader? What if I had some other disability (deaf, Parkinson's, EPILEPSY, color blind, etc) how would your site look to me? Why do I include Parkinson's I'm betting you're asking, well, try this experiment, shake your mouse and click on the links on your site while twitching your arm and barely able to hold on. If you use a tiny font size for links or have a link that's just as big as one character, hard isn't it? A good webmaster will have thought of deaf, blind, colorblind, and maybe epilepsy, and actually, if you're a good webmaster you won't have any flashing stuff on your site and no blinkentaags (blink tags for those who don't know of der blinkenlights). But did you think of people who have arthritis or carpal's tunnel (ironic, I know) or anything like that? Next is people who have severe ADD, they have programs that fade out your other windows to help you concentrate now, is your main panel getting their full attention? What about people with text browsers? This is rare, I know, but if I went right now to your site in Lynx, would I be fine getting around and using it all? Is your site friendly to those who use dial-up and don't accept cookies? Next is something that would seem to be a bit more common, people who customize their themes. I have mine set to a dark gray background with dark orange/pink text. This shows up fine. But if you set a background or text color and not the other my default still kicks in for the other. Which often means I'm looking at black text on dark gray, or my dark pink/orange text on a gray/white background. This makes it hard to read. There are people who don't set either but use images with white backgrounds. This looks odd. It makes them stand out more than the text/content and they seem so out of place. I know several people (the entire tech team at my college) who customize their OS. So set both a foreground and background color for your page, and if you're using CSS remember to set both of them for any element you customize one on.
Let's talk GIFs. They're nice and all, but if I'm scrolling down a page and run across one in a tutorial or anything at all I have to wait for it to restart. So where you can don't use them for anything but adding something shiny to your page.
That's all I can think of, please add anything you've thought of to the list so we can all learn of the odd things we never think of and rarely come across (before I made my background dark I hadn't thought of that, but now it's a pain reading pages a lot)
