DOM (for Document Object Model) is a way of conceptualizing the contents of a web page. You can think of this as an agreed-upon way of describing elements in the document, just as there is an agreed-upon way of describing houses on your street in English. We all acknowledge, for instance, that the long strip of tarmac dividing houses on one side from the other is called the “road”. Each house is numbered, but we might also address them by position: “The third house on the left”, for example, or “The house after the red house”. While each house or building will be entered in a slightly different way, we call that entry by the same name: “door”.

In just the same way, the DOM allows us to describe the content of web pages. DOM Level 0, created in 1997, was a basic page description language, but it grew rapidly in complexity. Both Netscape and Microsoft created completely different and incompatible DOMs in order to compete with each other, forcing developers to code for one browser or another if they wanted to add JavaScript to their pages.

In 1998, the W3C released DOM Level 1, a standardized API for referring to the content of web pages, and modern graphical browsers now support it to a high degree. The DOM has now become so ubiquitous that the scripting we are about to look into has been given yet another term: DOM Scripting.

The DOM is strongly reliant on validated, structured, semantic HTML code. Just as you might struggle to make sense of a city that has grown without any plan, reason, or building codes, JavaScript will have a very hard time working with a page that is invalid. For that reason, you should have a strong understanding of XHTML 1.0 Strict before attempting to add JavaScript to a page. Just like CSS, JavaScript loves clean code.