When Netscape started the Mozilla browser, it made the conscious decision to support W3C standards. As a result, Mozilla is not fully backwards-compatible with Netscape Navigator 4.x and Microsoft Internet Explorer legacy code; for example, Mozilla does not support as I will discuss later. Browsers, like Internet Explorer 4, that were built before the conception of W3C standards inherited many quirks. In this article, I will describe Mozilla’s quirks mode, which provides strong backwards HTML compatibility with Internet Explorer and other legacy browsers.