Every web page resides inside a browser window which can be considered as an object.

A Document object represents the HTML document that is displayed in that window. The Document object has various properties that refer to other objects which allow access to and modification of document content.

The way that document content is accessed and modified is called the Document Object Model, or DOM. The Objects are organized in a hierarchy. This hierarchical structure applies to the organization of objects in a Web document.

• Window object: Top of the hierarchy. It is the outmost element of the object hierarchy.
• Document object: Each HTML document that gets loaded into a window becomes a document object. The document contains the content of the page.
• Form object: Everything enclosed in the <form>...</form> tags sets the form object.
• Form control elements: The form object contains all the elements defined for that object such as text fields, buttons, radio buttons, and checkboxes.

Here is a simple hierarchy of few important objects:

DOM compatibility

If you want to write a script that uses the W3C DOM when it is available, and otherwise uses the IE 4 DOM if it is available, you can use a capability-testing approach that first checks for the existence of a method or property to determine whether the browser has the capability you desire. For example:

if (document.getElementById) { // If the W3C method exists, use it } else if (document.all) { // If the all[] array exists, use it } else { // Otherwise use the legacy DOM }