Progressive web apps (PWA) is the future of the web and igaming development. The technology is slowely grabbing attention of the American gambling companies who want to provide bettors with the easier mobile-friendly solutions (you don't need to download the app, it is installed automatically via your browser) and avoid sharing revenue (up to 30%) with Google and Apple. There are a few of them in 2019, more legal US sports gambling sites using PWAs to be expected in 2020.
US betting sites with progressive web apps (PWA):
Check gambling apps at PWA Store. It is a platform for progressive web apps, allowing users to browse and download progressive web apps on their iOS and Android mobiles and smartphones.