It makes a huge difference in support for web standards. There are many nice things that Firefox supports which Webkit does not. It also means that we have to lobby Apple to support standards, since we're not allowed to compete on merit (at least for engine-dependant features).

What's an example of a standard Firefox supports that Safari/webkit doesn't?

I work with cross-platform client-side JS libraries, though admittedly sticking to pretty vanilla/backwards compatible functionality, and the main thing I've seen that's wonky with Safari is the postMessage API. A few years ago, Firefox finally added the innerText API that many others had already adopted.

What other examples are there (this isn't a challenge, I am sure there are issues, just curious for my own knowledge!)

I should really write a script to generate this from MDN's browser compatibility data (https://github.com/mdn/browser-compat-data), but many APIs aimed at making browsers more competitive with native apps tend to lag behind or not get implemented at all in Webkit.

We only got working IndexedDB as of Safari 10 and Service Workers in Safari 11, both multiple years after support landed in stable Firefox. There's no support for WebXR, Web Push, or WebAuthn. Safari still can't display WebP images, show WebM (VP8/9) movies, or play Opus-formatted sound. Safari doesn't understand the current version of referrer-policy headers. Safari doesn't implement the String.prototype.matchAll() method. There's no support for getDisplayMedia(). Depending on how you count, we've been waiting seven years for getUserMedia() in Safari, and it's still not fully functional. Safari can't animate changes to CSS grid template rows/columns, or use overflow-inline / overflow-block media queries.

...it's a big weird grab bag of features, but a broad surface area nevertheless. And you have to wonder if some fringe APIs wouldn't be more popular if we could actually compete on bringing the full Web platform to iOS devices in a timely manner.

Epic. Thanks so much for sharing. I immediately wondered if I could do some caniuse.com magic to answer my own question.

As someone with an iPad Pro - Yes, definitely. There are so many sites that are just plain broken on mobile safari.

Actually, even Apple's own developer site is terrible on iOS. Views that should scroll to let you see content just plain don't work.

One thing that frequently annoys me about Safari/WkWebView is to tap on a link, see the link change in color so you know the browser has registered a tap event, and nothing happens. You have to tap again to actually navigate to a URL.