PWA actually do have a very rigid specification (from Google and ChromeOS) and a standard-in-practice from Apple that is pretty much the same.
And, of the requirements, the least understood is the having a service worker. In theory, they are meant to replace stand-alone apps and are meant to be run off-line as well. In theory. In practice, the support for it is very ... passive aggressive?. They want you to make PWAs because they sound like a great idea. But giving good support for them would kill app store sales so they don't really want you making any PWAs. So they do and don't want you to make them. But that is just my opinion :).
