But for a large subset of websites, there really isn’t THAT much difference between window.onload and DOMContentLoaded. If you’re merely adding an unobtrusive behavior layer to a traditional, content-driven website, chances are that good old window.onload will perform just fine. There’s usually little reason NOT to use DOMContentLoaded. But unless you are developing a desktop-style webapp or implementing progressive enhancement on a foundational level, it’s not really necessary. And in certain cases, it just flat-out won’t work.
An enthusiastic beta-tester discovered such a case when trying to initialize RSH’s dhtmlHistory object from DOMContentLoaded. It broke, and I immediately knew why: RSH relies on the ability of modern browsers to auto-save form data for the life of a session. RSH uses a hidden textarea to cache serialized Ajax application state and render the back button useful again. Internet Explorer doesn’t repopulate the cached value of that textarea until an instant after window.onload. If you try to access that cache during DOMContentLoaded, it simply isn’t there. This is true of both IE6 and IE7. Therefore, you need to wait for window.onload.
I’m not really sure there’s any big conclusion to draw from this example. As I said, it’s just interesting to me that in the space of a year or two, DOMContentLoaded has become a de facto standard. As we pile browser hacks on top of one another to push the web forward, sometimes they’re going to conflict. Luckily, we can always peek past the curtains and figure out what’s going on behind the scenes.