It means that the browser follows the standards set by the
W3C. For example, if a browser says that it is W3C HTML 4.01 compliant, it follows the
HTML 4.01 Specification. The specification mainly says what is
legal in HTML 4.01 and how the code should be treated by the browser.
Using a standards-compliant browser, you may notice that some pages look funny or don’t work at all. This is because some coders code their Web pages with non-standard code or expectations of non-standard behaviour. While most W3C-compliant browsers support some non-standard code and behaviour under certain circumstances, they don’t support everything. Therefore, you may have to open that page in a different browser (usually IE) to see everything as the coder intended. Don’t let that scare you though! For most people, it’s not really much of a problem.
EDIT: Small typo
EDIT 2: I’ve moved this topic.
Archived topic from Anythingforums, old topic ID:1627, old post ID:21638