☝️ If you don’t want to download the macOS 10.13.4 beta you can alternatively download Safari Technology Preview (> 48) for use with your current macOS install.
Great news indeed, yet as Maximiliano Firtman documented there are quite a few caveats right now. Some might get fixed, yet some others won’t. One of the bigger issues I find worrying is the fact that you can end up with different versions of one and the same PWA, depending on the context you are running it in:
On Safari, on each app’s Web view, and home screen web apps, the engine doesn’t share Service Workers nor the Cache Storage, which means the user might end with several copies of all the files for the PWA on the same device.