javascript

Chapter 10: The Document Object Model
This example shows that the values of documentElement , firstChild , and childNodes[0] are all the
same — all three point to the < html > element.
As an instance of HTMLDocument , the document object also has a body property that points to the
< body > element directly. Since this is the element most often used by developers, document.body tends
to be used quite frequently in JavaScript, as this example shows:
var body = document.body; //get reference to < body >
Both document.documentElement and document.body are supported in all major browsers.
Another possible child node of a Document is a DocumentType . The < !DOCTYPE > tag is considered to be
a separate entity from other parts of the document, and its information is accessible through the
doctype property ( document.doctype in browsers) as shown here:
var doctype = document.doctype; //get reference to < !DOCTYPE >
Browser support for document.doctype varies considerably, as described here:
IE — A document type, if present, is misinterpreted as a comment and treated as
a Comment node. document.doctype is always null .
Firefox — A document type, if present, is the first child node of the document. document
.doctype is a DocumentType node, and the same node is accessible via document.firstChild
or document.childNodes[0] .
Safari, Chrome, and Opera — A document type, if present, is parsed but is not considered a
child node of the document. document.doctype is a DocumentType node, but the node does
not appear in document.childNodes .
Due to the inconsistent browser support for document.doctype , it is of limited usefulness.
Comments that appear outside of the < html > element are, technically, child nodes of the document.
Once again, browser support varies greatly as to whether these comments will be recognized and
represented appropriately. Consider the following HTML page:
< !-- first comment -- >
< html >
< body >
< /body >
< /html >
< !-- second comment -- >
This page seems to have three child nodes: a comment, the < html > element, and another comment.
Logically, you would expect document.childNodes to have three items corresponding to what appears
in the code. In practice, however, browsers handle comments outside of the < html > element in the
following very different ways:
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