[Rant-Example] The goshdarn ticketing-system hijacks alt-f, so that instead of opening the File menu of my browser, and instead toggles the favorite-status of whatever ticket I happen to be viewing.
Corporate needs their Brandâ„¢ look precisely as specified in their expensive Style Guide. IBM wouldn't want the Google vibes of Android Material Design TextFields, I imagine.
Scratch beneath the visuals, and starker technical differences appear.
Safari on iOS (used to?) has a 350ms debounce delay on every tap / click, in case you want to do a multitouch gesture.
JavaScript (Frameworks) were the only way this arbitrary delay to user input could be reduced before 2015, when Apple finally released a native API for this.
https://webkit.org/blog/5610/more-responsive-tapping-on-ios/
Well, too many to have a single website be consistent across browsers.
But as a user I'm using one specific browsers, and I expect all websites be consistent for that browser.