Pjax is a technique that allows you to progressively enhance normal links on a page so that clicks result in the linked content being loaded via Ajax and the URL being updated using HTML5 `pushState`, avoiding a full page load. In browsers that don't support `pushState` or that have JavaScript disabled, link clicks will result in a normal full page load. The Pjax Utility makes it easy to add this functionality to existing pages.

It's easy to add pjax functionality to any page with just a few lines of code. You don't even need any special server-side logic. To include the source files for Pjax and its dependencies, first load the YUI seed file if you haven't already loaded it. Next, create a new YUI instance for your application and populate it with the modules you need by specifying them as arguments to the `YUI().use()` method.

This creates a new Pjax instance that, by default, listens for clicks on any link on the page that has a `yui3-pjax` class. When a `yui3-pjax` link is clicked, its URL will be loaded via Ajax and the loaded content will be inserted into the `#content` div, replacing its existing contents. If the loaded page includes an HTML `<title>` element, the current page's title will also be replaced with the new title.

One important thing to note is that since Pjax uses `XMLHttpRequest` under the hood, it can only load URLs from the same origin as the current page. This means the link URL must share the same protocol, port, and host as the current page. Pjax will ignore URLs it can't load, resulting in a full page load for those URLs.