What gets me is when websites don't work on Safari. Really? Like a significant portion of traffic is to iOS, it's a single browser you have to target, on millions of devices. And you couldn't even make it work there.
Apple makes it basically impossible to do proper testing for compatibility without buying a Mac or paying someone else that has a Mac to run your tests. Their entire app infrastructure is like this.
Another problem is that Safari is not compatible with the web. Like, I'm not going to prevent myself from using aspect ratios on images because some people made the bad decision of getting an inferior phone that locks them out of using good browsers. And many devs couldn't have known about it until the website was done and published to prod and iOS users started complaining about it because testing with Safari costs thousands of dollars in Apple hardware - and even if it was caught, it's still Apple's fault.
But ensuring full compatibility with all three major engines (those being gecko, blink and webkit) is unnecessarily hard, as they have their own subset of features
For example: Webkit does not support extending built-in HTML elements using WebComponent, but Gecko and Blink do support this feature. Or Chrome being the only browser that fully supports the View Transitions API. Or webkit's CSS vendor prefixes
The list goes on and on.
You could fix most of these issues by providing polyfills, but that increases the amount of files that you have to load in order to make a feature work on other browsers.
If only there was some sort of standard... Oh wait, there is one, W3C. Idk what they are doing tho.