Color
Browsers work with the system palette of each respective platform. It used to be that there were only 216 colors that were found on all platforms, so they made up the BROWSER-SAFE palette. Nowadays, almost everyone has monitors capable of showing millions of colors.
Resolultion
It used to be that the average monitor was 640 pixels wide. You would make your html page 480 x 350 so that it could be viewed fully by the smallest monitor, and that would include the margins of the browser. If an image on your page is larger than what the user's monitor can display, it automatically creates a scrolling bar. Keep in mind that text automatically wraps.
Now, the average monitor is more likely one that is set to 1090 x 800, or even larger than that, so we generally design for 900 by 550.
Some people prefer to keep their content smaller to allow for older monitors. (Also it is easier to read text if it is not too wide.)
As much as you can, try to find out what kind of monitors the people you are designing for will have. Remember to test your pages on all kinds of machines, all kinds of browsers, and all kinds of monitors. You can learn a lot by doing that, and come up with designs that work well in many different situations.