The DOM level 1 worked only with the Core and the HTML modules. While the extension level1 describes direct relationship between DOM and HTML tags, the extension level2 expands to all other document types, therefore it can be said that it is a language agnostic. The extension level 3 deals mostly with namespaces, inheritence and node names extensions and definitions.

DOM modules
NAME DOM LEVEL DESCRIPTION
DOM Core module level 1 The core module. Defines interfaces for manipulating document objects.
HTML module level 1 Explains changes in objects and methods specific only to HTML / XHTML documents.
CSS module level 2 Defines interfaces for document styling.
Events module level 2 Defines ways for managing events subscriptions.
Range module level 2 Describes the range of content and methods of accesing a document's tree.
Traversal module level 2 Defines interfaces for traversal of a document's content.
Views module level 2 Describes static or dynamic rendering of the document.
Load and Save module level 3 Loads content of a XML doxument to a DOM document or saves the DOM document to a XML one.
XPath module level 3 Describes the work withXPath expressions.

To check whether a DOM module exists or not within a browser, the DOMImplementation interface may be used in combination with the hasFeature() method.

var supported = document.implementation.hasFeature('Events', '2.0');

XML namespaces for L2 & 3
NAME (METHOD) DESCRIPTION
createAttributeNS() Creates an attribute in the given namespace URI document.
createElementNS() Creates an element in the given namespace URI document.
getAttributeNodeNS() Returns the attribute node from the given namespace URI.
getAttributeNS() Returns the attribute value from the given namespace URI.
getElementsByTagNameNS() Returns a NodeList of all children elements for the given namespace URI.
hasAttributeNS() Checks if the given attribute exist in the given namespace URI.
isDefaultNamespace() Checks if the given namespace URI is default or not.
lookupNamespacesURI() Returns the namespace URI with the given prefix of a node.
lookupPrefix() Returns the prefix with the given namespace of a node.
removeAttributeNS() Removes the attribute'a value from the given namespace URI.
setAttributeNS() Sets an attribute value for the given namespace URI.

Style module
NAME DEFINITON DESCRIPTION
cssText property Sets or returns the styles of a content as a string.
getComputedStyle() method Verifies what is the final computed style of an element.
getPropertyPriority() method Checks if the property is labeled as !important; or not.
getPropertyValue() method Returns the value of a property.
Item() method Reads the CSS property at the given position.
length property Returns the number (length) of properties used on an element.
parentRule property Sets or returns the CSS rule object with the information about the container.
removeProperty() method Remove the property specified in by the name.
setProperty() method Sets a new property (or modifes and old one) with its value and priority.

Style module - CSSStyleSheet interface
cssRules[] property An array collection containing the style sheets.
deleteRule() method Deletes a rule from the style sheet.
disabled property Checks if the style sheet is disabled.
document.styleSheets[] property Lists all style sheets available.
Element.sheet() / Element.styleSheet() method Reference a style sheet fetched from <link> or <style> element.
href property Verifies the URL of the style sheet if external (<link>)
insertRule() method Inserts a rule from the style sheet at the given index.
media property Lists the media types supported by the style sheet.
ownerNode property Indicates which element owns the styles that it refers to.
ownerRule property Returns the styles if they were imported with the @import.
parentStyleSheet property Indicates which style sheet was included with the @import rule.
title property Indicates the title attribute of the ownerNode.
type property Indicates the type of the style sheets.

Style module - CSSRule interface
cssText property Returns the CSS rule as a text.
deleteRule() method Deletes an existing rule.
insertRule() method Inserts a new style rule at the given position.
parentRule property Returns the parent containing rule if existing.
selectorText property Returns the selector text for the rule.
style property Returns the CSS style object that sets ot gets a specific value for the rule.
type property Indicates the type of the rule.

DOM element dimensions
NAME DEFINITON DESCRIPTION
clientHeight property Returns an element content area height together with the top and bottom padding.
clientWidth property Returns an element content area width together with the left and right padding.
offsetHeight property Returns an element layout's height together with the horizontal bar, top and bottom border.
offsetLeft property Returns an element position from the left edge to inside the left border.
offsetTop property Returns an element position from the top edge to inside thetop border.
offsetWidth property Returns an element layout's width together with the scrollbar, left and right border.
scrollHeight property Returns the height of the content without scrollbars.
scrollLeft property Returns the position of the content from the left edge.
scrollTop property Returns the position of the content from the top edge.
scrollWidth property Returns the width of the content without scrollbars.

Traversal module
NAME INTERFACE DEFINITON DESCRIPTION
acceptNode() NodeFilter method Determines if node should be filtered before passing it a NodeIterator or a TreeWalker.
createNodeIterator() NodeIterator method Creates a node iterating object.
createTreeWalker()  TreeWalker method Creates a tree walking object.
detach() NodeIterator method Releases the node object.
firstChild() TreeWalker method Finds the first child of the current node.
lastChild() TreeWalker method Finds the last child of the current node.
nextNode() NodeIterator, TreeWalker method Finds the next node relative to the current node.
nextSibling() TreeWalker method Finds the next sibling node relative to the current node.
parentNode() TreeWalker method Finds the parent node of the current node.
previousNode() NodeIterator, TreeWalker method Finds the previous node relative to the current node.
previousSibling() TreeWalker  method Finds the previous sibling node relative to the current node.

Range module
NAME DEFINITON DESCRIPTION
cloneContents() method Returns range data to be used elsewhere.
commonAncestorContainer property Finds the highest parent container that has starContainer and endContainer as descendants.
CompareBoundaryPoints() method Compares current range with the given one.
createRange() method Creates a new range.
deleteContents() method Deletes range data from original range.
detach() method Releases (cleans up) the created range.
endContainer property Selects the node within which the range ends.
endOffset property Selects the offset within the ending node of a range.
extractContents() method Returns range data to be used elsewhere and deletes them from original range.
insertNode() method Inserts a node to a range.
selectNode() method Select a node and its children within a range.
selectNodeContents() method Select a node's children.
setEnd() method Sets the end position of a range given by the end node and end offset.
setEndAfter() method Sets the end position of a range after the given node.
setEndBefore() method Sets the end position of a range before the given node.
setStart() method Sets the start position of a range given by the start node and start offset.
setStartAfter() method Sets the start position of a range after the given node.
setStartBefore() method Sets the starts position of a range before the given node.
startContainer property Selects the node within which the range starts.
startOffset property Provides the offset where to start the conatiner.