Well for starters, no one else is allowed to publish a browser with their own engine on the AppStore, and they've hampered sideloading for years. In a vacuum, it might be reasonable to block any third-party browser engines, or to put something special in their own that no one else can use on the phone, but combining those is just intentional sabotage. And yes, I know that this specific CSS property isn't all that important, but having an argument about how much they're allowed to intentionally sabotage other browsers on their phones is at best a misguided distraction from the point that they shouldn't be doing it in the first place (and not a particularly good defense either; try arguing with a cop who pulls you over for speeding that the law you broke wasn't really that important and see if it gets you out of the ticket).