There are a specific set of requirements for a web app to be a web app. It's significantly more than just rolling out a random pretty website.
As a developer there are specific tests to pass before it's considered an app.
To add to the complexity, different browsers have (slightly) different requirements.
Source: I did this once. It wasn't fun. My proof of concept died quickly. I may revive it at some point.
That said, it's significantly simpler to do this than write an app that runs on MacOS, Linux, Android, iOS and Windows.