In some ways, adding Ajax to an existing site is much easier than developing an application with Ajax as a component from the beginning. First of all, you already have your script-free application versions, and you won't be tempted to skip this essential component.
After doing the preliminary development planning discussed in Chapter 1, and after you have a good idea of where you want to use Ajax (and why), you now have design issues related purely to the development process.
For instance, rather than having the CSS for an entire site in one file, we could split it out into separate CSS files for individual effects, and then load each of the CSS files only when the page needs a particular effect. Of course, the more we do this, the more HTTP requests are made, so another approach is to manually merge the CSS for all the effects into one ...