[lhorie]

Haven't dug too deep, but my understanding is that this doesn't bind event handlers just in time, but instead sets up event delegation from a tiny blocking bootstrapping script, to attach a top-level event handler that catches all events as soon as the first chunk of HTML streams in.

In addition, it sets up an intersection observer. Then depending on when an event happens, it might require downloading that one event handler piecemeal if the event occurred early enough during page load, or if the event is late enough, the action happens instantaneously because the intersection observer already downloaded the handler in anticipation that the user would interact with the element, it being visible and all.

The trade-off is that the download of every other JS thing effectively gets deferred due to fragmentation of how JS gets loaded in the page, but the cleverness of the trade-off is that in typical scenarios, most of that deferred code is not going to be activated by the user in the first place (or at least not in quick succession so as to overload network).