Compatibility
Designing web pages that look the same no matter how they are viewed is not as straightforward as perhaps it ought to be. Not every PC uses the same screen resolution, can display the same colours or has the same installed fonts. And of course, not everyone will use a PC.
Often the biggest problem is the range of browser software in use. Most PC owners are familiar with Microsoft's Internet Explorer, but other browsers such as Firefox, Opera and Safari are commonly used, and these have the potential to present pages in different ways. Even different versions of the same browser can exhibit alternative behaviour.
To alleviate the problem, software developers are encouraged to adhere to code specifications published by the Worldwide Web Consortium (W3C). By writing website code that complies with these standards web designers can eliminate errors and help to ensure that pages appear the same across the broad range of available browser software.
Even so, not all software packages interpret the specifications equally and differences may still be apparent. Some designers still regularly make the mistake of assuming that pages that appear satisfactory in Internet Explorer will be acceptable in Firefox. Layouts that don't quite fit together, overlapping page elements or text that spills out of columns and boxes are all symptoms of this. It's essential, therefore, that pages are thoroughly tested in a range of browsers to ensure that they display correctly.
For more information on W3C's standards:
HTML 4.01 - http://www.w3.org/TR/html401/
XHTML 1.0 - http://www.w3.org/TR/xhtml1/
CSS2 - http://www.w3.org/TR/REC-CSS2/
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