"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
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.
"I'll also cover nonstandard technologies, such as
XMLHttpRequest and rich text editing, that Mozilla does support
because no W3C equivalent existed at the time..."