The <!DOCTYPE> tag at the top of the document is instantiated into a Doctype object and a reference to it is stored here. This tag describes a DTD statement that identifies the type of document.
Here is a properly formed DTD statement:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 FINAL//EN">
This is represented by an object whose type is a single exclamation mark. As this tag should be on the first line of a document, the object will be at index 0 in the document.all array.
Support for this is still somewhat patchy as of Netscape 6.0 and MSIE 5.5. Netscape 6.0 returns a DocType object (note the class name capitalization) but its string value is not defined. MSIE returns an undefined value.
There is apparently no access to this value in Netscape and MSIE only provides limited access.