The main way of accessing specific parts of an already existing web page is by using the getElementById method attached to the document. This particular method provides us with a way to access a particular container within the document. That container must already exist at the time that we try to access it. (We will look at adding to the page in later tutorials).
The first thing that we need to do in order to be able to access a specific container using this method is to mark the container so that we can uniquely identify which container it is that we want to refer to. We do this by adding an extra attribute to the HTML tag that we want to identify. his extra attribute is id (hence the name getElementById).
var content = document.getElementById('content');
We'll look more at how to access the properties of a container object in the next tutorial.