[CharlesW]

The most significant effect is that experimental and non-standard PWA capabilities aren't reflected in the primary score. You can see raw/unweighted scores by hovering over the primary score. Chrome wins handily if you count experimental/non-standard features.

For standards-based features I used a 4-tier model, described about halfway through the README (which I should also add to About):

    ┌────────┬──────────────┬───────────────────────────────────────────────┐
    │ Weight │ Tier         │ Rationale                                     │
    ├────────┼──────────────┼───────────────────────────────────────────────┤
    │ 3.0    │ Core PWA     │ Prerequisites for production PWAs (6 features │
    │        │              │ features: Web App Manifest, Service Workers,  │
    │        │              │ Caching, HTTPS, etc.)                         │
    ├────────┼──────────────┼───────────────────────────────────────────────┤
    │ 2.0    │ Important    │ Enhance PWA functionality (18 features: Push  │
    │        │              │ API, Add to Home Screen, Offline Support,     │
    │        │              │ Display Modes, etc.)                          │
    ├────────┼──────────────┼───────────────────────────────────────────────┤
    │ 1.0    │ Standard     │ Default weight (94 features)                  │
    ├────────┼──────────────┼───────────────────────────────────────────────┤
    │ 0.5    │ Experimental │ Nice-to-have capabilities (43 features:       │
    │        │              │ Sensor APIs, Bluetooth, NFC, AR/VR, etc.)     │
    └────────┴──────────────┴───────────────────────────────────────────────┘

This weighting turns out to be reasonably conservative. For example, if you hover over the score for Firefox (the largest benefactor), you'll see that it bumps Firefox's score by 5.

I'm very open to feedback. This is a sincere attempt to quantify vendors' PWA support.