Everyone of your site visitors has a different computer, a different size monitor, different settings and preferences for everything from text size to screen resolution, different modem speed (bandwidth) and more. The myriad possible combination of variables makes each and every user’s experience different. This, plus the fact that designers and developers often have more advanced equipment than the common person, makes creating a uniform design almost impossible.

The good news is that the sites you create don’t need to be compatible with all platforms and browsers everywhere. They just need to be compatible with the hardware and software that most of your intended visitors are using. A reasonable goal is to accommodate at least 90% of your site’s users. Here are the three most important variables to take into consideration:

1. Hardware (PC, Mac, or others) and Software (browser, such as IE, Netscape or others)

Each system and Web browser has its own quirks, some of which affect the following:

• Offset - The space between the edge of the browser window and where the content begins.
• Canvas size - When a browser window is set for 600 x 800 screen resolution, you don’t actually have that amount of pixel space available for content. You must allow space for browser toolbars, which are generally in default mode, making the windows people are actually using much smaller.
• Text size - Although there is very little variation in HTML text size among browsers, there are differences from platforms to platform. You can be fairly assured of good display with a basic font set including Verdana, Arial and Helvetica. But users can change the default settings for type size and other graphic elements, so you must accommodate for that. 
• Color- A major difference between platforms is the way they handle gamma, or the amount of displayed brightness. That means that any graphic you design on a Mac will appear darker and muddier when viewed on a PC. Conversely, any graphic you design on a PC will look somewhat washed out on a Mac.

When designing for specific user groups, find out how up-to-date their hardware and software is. If they’re sophisticated users, they may have the latest version. But remember that not everyone rushes into browser upgrade every time a new one is released. And unless you know that your users are using another platform, such as Linux or Unix, don’t waste time designing for (or conducting usability tests on) them.

2. Screen resolution

Screen resolution refers to the dimensions, in pixels, of your screen's display. (This has nothing to do with monitor size. Any size monitor can be set to almost any resolution.) The most common dimensions are 640 x 480, 800 x 600 (the current standard), and 1,024 x 768. It's important to find out which screen resolution the vast majority of your target audience is using because it affects the layout of your page. The most crucial content must display within their screen sans scrolling—in other words, above the fold, that most valued area of real estate—so they are sure to see it. The fold also wraps around the right edge, so anything hidden off to that side would also be below the fold. You can place less important content to the right of or below the fold.

3. Bandwidth

Bandwidth refers to the pipe: it’s the connection speed through which the Internet arrives at the user’s computer. Some users have high (fast) bandwidth via DSL, a cable modem, T1 or T3 connections. Some have low (slow) bandwidth, via dialup modems.

The number one complaint on the Web is that pages download too slowly, and while there are many factors that contribute to how long a page takes to download, one of the main factors is the connection speed of your users. Plus, many things can slow down connect speed. A 56 kbps modem may, in reality, not get above 48 kbps. Also, there may be traffic on the Internet, which slows downloading time. So you must know what kind of bandwidth the majority of your users have and then remember that it’s probably a little slower.

Balance technical flashiness with a healthy concern for your viewers' bandwidth capacity. While high speed connections are available to an ever-increasing number of users, especially business users, as of 2003, there are still a lot of people dialing up on standard 56-Kbps (or lower) modems.