There are a few different schools of thought when it comes to enhancing your HTML pages with JavaScript. Let's talk about some of the things we should consider before we dive into the cool stuff.

Progressive enhancement and graceful degradation are essentially two sides of the same coin. They both mean that our page with its impressive JavaScript animations and special effects will still work for users who have less capable browsers or devices. Graceful degradation means that we create our special effect and then make sure it fails gracefully if JavaScript is not enabled. If we take the progressive enhancement approach, we'll first build out a bare bones version of our page that works for everyone, and then enhance it by adding on our JavaScript special effects. I tend to favor the progressive enhancement approach.

Why should we care about users who don't have JavaScript enabled? Well, one of the Web's biggest users—search engines—do not have JavaScript capabilities. When search engines are crawling and indexing your pages, they will not have access to any content that's being loaded into your pages by JavaScript. That's often referred to as dynamic content, and it won't be indexed or found by search engines if it can't be reached with JavaScript disabled.

We're also in an era where we can no longer count on users accessing the web pages we build with a conventional desktop or laptop computer. We're quick to think of smart phones and tablets as the next candidates, and while they are very popular, they still only account for a tiny fraction of Internet access.

People are accessing the Web from gaming consoles, e-book readers, Internet-enabled televisions, a huge variety of mobile devices, and probably hundreds of other ways. Not all of these devices are capable of executing JavaScript—some of them don't even have color screens! Your number one priority should be making sure that your content is available to anyone who asks for it, no matter what device they happen to be using.

To accomplish this task of making our content available to as wide an audience as possible, we have to think of our web pages in three separate and distinct layers: content, presentation, and behavior.