Here's a really simple example of something that would benefit standard websites, not just complex web applications: image lazy loading.
<img src="..." loading=lazy>
Sure you can handle this with JavaScript, etc. The point is that this is one of countless papercuts - standard, useful behaviors that work everywhere but Safari.
WebKit already has experimental support for this, just needs to be enabled in Dev tools. Which means it will be rolling in to Safari and other WebKit browsers soon.