Main?Introduction
Before you publish your hard work, you should test your site in all conditions. Sure, everything looks fine on your computer, with your settings, but not everyone will have those settings.
This guide will help you remember all the different conditions your site could be viewed under.
For the condense version of this guide, checkout the Web Page Testing Checklist.
Browsers
A majority of your visitors will have the most current versions of Internet Explorer, so if you test your web page in nothing else, test them in there.
But, you also need to check your website on as many versions of Netscape Navigator as possible, but mostly 4+. When you first start designing your own web pages Navigator may seem like your mortal enemy, but if your only average 100 - 500 hits a day, don't sweat it too hard, because Netscape isn't nearly as popular as Internet Explorer.
Checking your design in the AOL browser may be worth your time as well, as the AOL browser tends to do some funky things to websites, and I'm pretty sure that most versions decrease the quality of images for better load time.
It's also possible that your web site will be viewed on a TV Screen, due to web TV and certain video game consoles that allow you to connect to the Internet. Television browsers tend to make fonts a lot larger (so they can be viewed from a distance) and the general appearance will be a bit fuzzy.
The Opera Browser is also commonly used, and may be worth installing on your computer.
It's also highly unlikely that you will have to worry about this, but if you ever receive the opportunity, checking your design on old text based web browsers like DOS Lynx might be worth your time. Unless you have a large corporate website though, don't even sweat this.
Screen Resolutions
Most people always change the resolution of their computer. If you are a windows user, you can do this by going to your control panel, selecting display, clicking the settings tab, and adjusting the screen area.
Be certain that you always check your design in 800 x 600, and 1024 x 768. If you keep all the major content of your website in an area that does not exceed 700 pixels wide, by 400 pixels in height your design should always look well in these two resolutions. (Mildly empty in 1024 x 768, but should still be fine).
Checking your site on 640 x 480 and 1280 x 1024 might be wise as well.
In the display settings, found in the control panel folder, you can adjust the amount of colors your monitor displays as well. You typically get three settings: 256, 16 bit, and 32 bit. Make sure your design looks well in all these settings.
Operating Systems / Different Computer Types
The more Operating Systems you can check your design on, the better.
Most visitors will have current versions of Windows for the PC, but don't forget to check your design on older Windows Operating Systems. The Mac is starting to gain more popularity, so you will need to check your design on a Macintosh as well.
Testing things on UNIX Machines would be wise, but not vital.
Speed
When you first upload your website, try to find older computers with 14.4 Kbps, or 28.8 Kbps modems to test the load times. If it loads too slow, you need to reduce the number of graphics, and multimedia you have. Also try to reduce the size of your pages, and divide the content up.
Printed
If you are providing content on your web page that is likely to be printed, then run a test print to make sure everything prints okay. Some printers will print the content of a website on a white background, but with the same font settings. This could be a problem if you have white as your font color, or a small font.
If your page is likely to be printed, stick to easy to read black fonts on white backgrounds, or provide a separate printable version of your document.
Misc
Some browsers may not support certain elements of your web site, such as CSS or JavaScript. Before you complete your website, remove the CSS and JavaScript from it. If removing the CSS / JavaScript ruins the presentation of your web site, you may need to reconsider the design.
You will also want to check the W3C out, as they make the latest HTML and web development standards.
Because some people turn images off, for faster loading, on thier browsers, you may wish to check your website without it's images. It's recommnded that you always use the alt attribute so people not able to view images will at least know what they are about.