They don’t all implement the same set of standards equally. The one that comes immediately to mind is one that comes up frequently as “hey do you know your website is broken on iOS” on HN is when someone is demoing some really interesting web-tech that requires SharedArrayBuffer (since it enables thread-like behavior in JavaScript and is super useful also for performant interaction with the GPU).
Browsers can of course be non-conformant on other interesting issues. Imagine if a browser came out that allowed the Unity Web Player to work. Instantly, people wouldn’t need to use the App Store to purchase games. Instead, they could just use the browser.