That seems to be developers making the web browser less feature rich than desktop app. So browsers like Firefox, Brave, Opera and Edge will have less feature than the desktop browsers.
Not sure what you mean by webkit but browsers like Firefox, Brave, Opera and Edge are plain like you see on the iPhone not some thing you see on iPad yes it the same app on both devices.
Android Firefox app is way more feature rich than what you get on iOS or iPadOS it just developers are making the web browser less feature rich not sure what Apple can do here. If developers are just lazy and giving very basic browser than what you get on desktop or laptop running Windows.
Yes on my desktop running Firefox I can make all websites into dark mode, change website text, block scrips running, download videos from most website, take screenshots, remove items from website, block ads, download YouTube videos, block malware, have themes browser so on. Done of that is possible with iOS or iPadOS but some this is possible with Android.