Cover by Steve Souders

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The Best of Both Worlds

Even if all the factors point to inlining, it still feels inefficient to add all that JavaScript and CSS to the page and not take advantage of the browser's cache. Two techniques are described here that allow you to gain the benefits of inlining, as well as caching external files.

Post-Onload Download

Some home pages, like Yahoo! home page and My Yahoo!, typically have only one page view per session. However, that's not the case for all home pages. Yahoo! Mail is a good example of a home page that often leads to secondary page views (pages that are accessed after the initial page, such as those for viewing or composing email messages).

For home pages that are the first of many page views, we want to inline the JavaScript and CSS for the home page, but leverage external files for all secondary page views. This is accomplished by dynamically downloading the external components in the home page after it has completely loaded (via the onload event). This places the external files in the browser's cache in anticipation of the user continuing on to other pages.

The post-onload download JavaScript code associates the doOnload function with the document's onload event. After a one-second delay (to make sure the page is completely rendered), the appropriate JavaScript and CSS files are downloaded. This is done by creating the appropriate DOM elements (script and link, respectively) and assigning the specific ...

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