Is there a way to understand how many of the Web Platform Tests are for features that are "stable" (say for X years) across all major browsers? I see Chrome has a much higher number than the others, which seems to indicate that many of the tests may be for features that are not compatible across browsers.
It's hard to tell exactly, but if you look at the numbers you can see that, for example, Firefox and Safari don't support WebUSB or APIs for accessing things like the accelerometer or ambient light sensor of the device the browser is running on the device the browser is running on (this is by choice I believe) or things like Scalable Video Coding for WebRTC (rtc-svc).