I was writing a feature where I was on the client side writing the iframe code for a video oEmbed. This causes a delay while the content is loaded in the iframe. It behaves like an AJAX request in a sense. I discovered that the iframe raises a 'load' event when the content finishes loading. I got that from this stack overflow answer about the embed tag. I tested it in Safari, Chrome, Firefox, and Edge - it works. I expected that this was a detail I missed in Mozilla's iframe documentation, but I couldn't find it. So, I am at a loss as to how a developer is supposed to know details like this. I was lucky to have found it, but it makes sense that there should be an event to hook for is action.
This is documented in the HTML spec, which probably means that I need to spend more time referencing that directly.
This just reinforces my belief that a good deal of developing for the front end isn't as much about the JavaScript language as it is about knowing the ropes of using the various web APIs that are available. In this case it would mean just knowing that a feature exists. The browser gives us a lot for free and it makes sense to be able to take the best advantage of that.
Actually looking at the HTML spec it seems that even the img element fires a load event when the content has been successfully loaded.
Reference: HTML Living standard