In reality, your web pages need to look the same on four browser brands: Microsoft Internet Explorer, Mozilla Firefox, Safari, and Opera. The significance of cross-browser compatibility with the different brands comes in the same order as the listing above.
The Acid2 test
The Acid2 test is a test case designed to measure a browser's adherence to accepted web standards. It is available at http://www.webstandards.org/files/acid2/test.html
The Acid2 test is expected to render correctly on any browser that follows the W3C HTML and CSS 2.0 specifications. When a browser is 100% standards compliant and passes the test, it displays a stylized smiley face, displayed as image below. The nose flashes blue on mouseover.
The first browser to pass the test was Safari for MacOS. The most popular browsers -- Internet Explorer and Firefox, fail to render it properly, and thus the Acid2 test is more of a recommendation for future version of those browsers.
The IE box model bug
The Internet Explorer box model bug is probably the best-known software bugs in implementating Cascading Style Sheets . It affects versions of Microsoft's Internet Explorer web browser for Windows up to version six. Internet Explorer 6 and 7 are not affected in their standards-compliant mode, only in the-so called "quirks mode," which kicks in when the browsers fails to match content and DTD.
When the bug is manifest, Internet Explorer includes the padding and borders within the specified width or height, resulting in a narrower or shorter box.