Are you sure that all of your website visitors see the same thing when coming to your URL? There are many different viewing possibilities such as browser, screen resolution and individual PC color and contrast settings.  View your site in various combinations to be sure that everyone receives the same experience.

Strive for cross-browser compatibility
The same web page can appear differently when viewed in different browsers.  Internet Explorer, Mozilla Firefox, Safari, and others each display code differently, so you must be careful to view pages in each before going live.

Usually, you can find common ground by utilizing code (stylesheets/CSS) that all of the major browsers recognize, but there may be cases that you should create distinct versions of the same page(s).  Issues that may arise are the use of features such as image maps, frames, javascript, java applets, or operating system-specific controls.  Test in as many environments as possible!

Layout and design for multiple screen resolutions
As mentioned earlier, another challenge is to make your pages attractive at various screen resolutions.  The vast majority of PCs display 1024 x 768 pixels while most laptops are set to 1280 x 1024.  There are still a number of surfers using 800 x 600.  Setting pages to show a set page width of 800 pixels will display the same on each of these, and if centered, will simply have larger left and right margins to those with greater resolution.

Validate your HTML code
After designing pages, it’s a good habit to get into using HTML Validator software to reduce errors that could cause display problems in some browsers and penalize you in some search engines.