The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents.Objects in the DOM tree may be addressed and manipulated by using methods on the objects. The public interface of a DOM is specified in its application programming interface (API). The history of the Document Object Model is intertwined with the history of the "browser wars" of the late 1990s betweenNetscape Navigator and Microsoft Internet Explorer, as well as with that of JavaScript and JScript, the first scripting languages to be widely implemented in the layout engines of web browsers.
JavaScript was released by Netscape Communications in 1996 within Netscape Navigator 2.0. Netscape's competitor,Microsoft, released Internet Explorer 3.0 later the same year with a port of JavaScript called JScript. JavaScript and JScript let web developers create web pages with client-side interactivity. The limited facilities for detecting user-generated events and modifying the HTML document in the first generation of these languages eventually became known as "DOM Level 0" or "Legacy DOM." No independent standard was developed for DOM Level 0, but it was partly described in the specification of HTML4.
Legacy DOM was limited in the kinds of elements that could be accessed. Form, link and p_w_picpath elements could be referenced with a hierarchical name that began with the root document object. A hierarchical name could make use of either the names or the sequential index of the traversed elements. For example, a form input element could be accessed as either "document.formName.inputName" or "document.forms[0].elements[0]."
The Legacy DOM enabled client-side form validation and the popular "rollover" effect.
In 1997, Netscape and Microsoft released version 4.0 of Netscape Navigator and Internet Explorer respectively, adding support for Dynamic HTML (DHTML), functionality enabling changes to a loaded HTML document. DHTML required extensions to the rudimentary document object that was available in the Legacy DOM implementations. Although the Legacy DOM implementations were largely compatible since JScript was based on JavaScript, the DHTML DOM extensions were developed in parallel by each browser maker and remained incompatible. These versions of the DOM became known as the "Intermediate DOM."
在1997年，网景和微软分别发布了Netscape Navigator4.0和Internet Explorer，增加对Dynamic HTML(DHTML)的支持，一个已经加载的HTML对象其功能可以改变。DHTML需要的基本文本对象的扩展在传统的DOM就可以得到。虽然，由于JScript基于Javascript，传统的DOM实现有了很大的兼容性，但是DHTML DOM扩展是被每个浏览器场上并行的发展，仍然不兼容。这些版本的DOM被称为“中级DOM”。
The World Wide Web Consortium (W3C), founded in 1994 to promote open standards for the World Wide Web, brought Netscape Communications and Microsoft together with other companies to develop a standard for browser scripting languages, called "ECMAScript." The first version of the standard was published in 1997. Subsequent releases of JavaScript and JScript would implement the ECMAScript standard for greater cross-browser compatibility.
After the release of ECMAScript, W3C began work on a standardized DOM. The initial DOM standard, known as "DOM Level 1," was recommended by W3C in late 1998. About the same time, Internet Explorer 5.0 shipped with limited support for DOM Level 1. DOM Level 1 provided a complete model for an entire HTML or XML document, including means to change any portion of the document. Non-conformant browsers such as Internet Explorer 4.x and Netscape 4.x were still widely used as late as 2000.
DOM Level 2 was published in late 2000. It introduced the "getElementById" function as well as an event model and support forXML namespaces and CSS. DOM Level 3, the current release of the DOM specification, published in April 2004, added support for XPath and keyboard event handling, as well as an interface for serializing documents as XML.
By 2005, large parts of W3C DOM were well-supported by common ECMAScript-enabled browsers, including Microsoft Internet Explorer version 6 (2001), Opera, Safariand Gecko-based browsers (like Mozilla, Firefox, SeaMonkey and Camino).
W3C发布ECMAScript后，开始着手制定标准化的DOM。被称为DOM Level1的最初的DOM标准，在1998年底成为W3C的建议。大约在同一时间，IE5.0附带了有限的对DOM Level 1的支持。DOM Level 1提供一个完整的HTML或XML文档模型，其中包括改变文档任何部分的方法。在2000年底，不符合标准的浏览器，像IE4.x和网景4.x仍然被广泛使用。DOM Level 2 在2000年底发布。它引入了“getElementById”函数，以及事件模型和XML命名空间和对CSS的支持。DOM Level 3是当前的DOM标准，发布于2004年4月，增加了对XPath和键盘事件，以及序列化文件像XML的支持。到2005年，W3C DOM的大部分能被共同支持ECMAScript的浏览器很好的支持，包括微软的IE6（2001），Opera，Safari，以及基于Gecko的浏览器（像Mozilla，Firefox，SeaMonkey和Camino）。
To render a document such as an HTML page, most web browsers use an internal model similar to the DOM. The nodes of every document are organized in a tree structure, called the DOM tree, with topmost node named "Document object". When an HTML page is rendered in browsers, the browser downloads the HTML into local memory and automatically parses it to display the page on screen. The DOM is also the way JavaScript transmits the state of the browser in HTML pages.
Because DOM supports navigation in any direction (e.g., parent and previous sibling) and allows for arbitrary modifications, an implementation must at least buffer the document that has been read so far (or some parsed form of it).
Web browsers rely on layout engines to parse HTML into a DOM. Some layout engines such as Trident/MSHTML and Presto are associated primarily or exclusively with a particular browser such as Internet Explorer and Opera respectively. Others, such as WebKit and Gecko, are shared by a number of browsers, such as Google Chrome, Firefox and Safari. The different layout engines implement the DOM standards to varying degrees of compliance.
原文来自维基百科：
| http://en.wikipedia.org/wiki/Document_Object_Model |
