Web content must be rendered through WebKit as per the App Store Review Guidelines (https://developer.apple.com/app-store/review/guidelines/). This is supposed to be enforced by App Store Review by inspection. In addition to this, there are technical reasons why your “browser” will be inferior to Apple’s web view performance-wise, mainly having to do with apps that generate code dynamically in-process needing entitlements that prevent submission to the review process.