Many believe it's impossible or prohibitively expensive to develop an AJAX web application that also performs well from the perspective of SEO, which often sacrifices search engine crawlability. However, by using XSL (a templating language for XML), the HTTP Accept header, and API-driven development, you can now present an appropriately formatted document regardless of the technology being used to access the site. Additionally, applications developed in this manner perform well across many browsers (even lynx) and with assistive technologies for the disabled.
You're about to dive into the de facto introduction to Progressive Enhancement for AJAX, exactly how the W3C recommends you do it!
Typically, web applications use a server-side template language. The controller usually passes a bunch of variables to a view that is rendered into HTML and then served to the user. With API-driven development, you have the controller pass back the same data as an XML document straight out to the browser. The XML document has a special tag that tells the browser to grab a template and transform the data and the template into an HTML5 document. All modern browsers support XSL now. Plus, all the pages' template rendering occurs within the client--freeing your server from having to render those documents itself, caching commonly used templates semi-permanently in the browser cache.