Web Design - Important Factors to keep in mind

The viewing area of web is always unpredictable and because of that web designers has to keep in mind few factors before they call their site "complete":
1) Operating System (Windows, Mac)
2) Screen Resolution (800 x 600, 1024 x 768, 1440 x 900)
3) Browser with various version (IE 6.0, IE 7.0, Firefox, Safari)