As anyone that's done web programming can tell you, browser compatibility can be a headache. When I first threw html on the web 10 years ago, trying to get pages to render the same way in IE and Netscape was sometimes a challenge because W3C standards weren't followed consistently. Even now, compliance is an issue. Firefox, IE, Chrome, and Safari all take liberties with some standards, and the result can be a nightmare for developers.
Best practices dictate development in compliance with standards, and then managing any big problems with compatibility if required. Practically, some developers change style sheets for each browser or simply design their sites to look good in the browser most of their readers use and make tweaks when required for other browsers.
Acid2 is a test page developed to highlight the inconsistent adherence to W3C standards:
Acid2 is a test page published and promoted by the Web Standards Project to expose web page rendering flaws in web browsers and other applications that render HTML. It was developed in the spirit of Acid1, a relatively narrow test of compliance with the Cascading Style Sheets 1.0 (CSS1) standard, and was released on April 13, 2005. Like Acid1, an application passes the test if the way it displays the test page matches a reference image.
Acid2 tests aspects of HTML markup, CSS 2.1 styling, PNG images, and data URIs. The Acid2 test page will be displayed correctly in any application that follows the World Wide Web Consortium and Internet Engineering Task Force specifications for these technologies. These specifications are known as web standards because they describe how technologies used on the web are expected to function.
While at the time of Acid2's release no web browser passed the test, Acid2 was designed with Microsoft Internet Explorer particularly in mind. The creators of Acid2 were dismayed that Internet Explorer did not follow web standards and because of this Internet Explorer was prone to display web pages differently from other browsers. When such a discrepancy between browsers is encountered, web developers spend time tweaking their web pages in order to make the pages be displayed correctly in different browsers. Acid2 represented a challenge to Microsoft to bring Internet Explorer in line with web standards, making it easier to design web pages that work as intended in any web browser.
Check out the wikipedia article on Acid2 for some pictures of how the most popular browsers don't adhere to standards, and check out the site itself to see how the browser you're using measures up.
Updated: Firefox Beta Scores 93 On Acid3 Test (via Slashdot)