While I am happy to see that some kind of lazy-loading is coming to Core and it is a good feature to have. I’m just not that sure about the timing of this. Despite the nice wording around it in the post, this is not yet part of WHATWG specification and only one rendering engine actually supports it. So this seems like a bit of putting the wagon before the horse. One might also see this as using the weight of WordPress to push through a feature that has not seen a wide adoption among browser vendors, yet. Firefox is nowhere near supporting this yet, for example.