Safari 26 brings a smoother, more app-like feel to many websites on iPhone and...

https://mike-wiki.win/index.php/What_Does_%E2%80%9CBrowser-First%E2%80%9D_Actually_Mean_for_a_Mobile_Service%3F

Safari 26 brings a smoother, more app-like feel to many websites on iPhone and iPad. When you add a site to your Home Screen, it now behaves more like a native app with fewer browser elements visible