Given how vast the modern web specifications are, you could really be 99% compliant but provide a pretty mediocre experience to your users, or vice versa.
For example, I use Firefox as my main browser, and the lack of e.g. proper WebAuthN support on macOS has a much, much higher impact to me (and is making me consider switching), whereas I couldn't care less about e.g. WebUSB (which I do occasionally use, but I'm fine switching to Chrome for that use case).
To get an accurate picture, you'd have to somehow weigh all of these various standards and features in a way that not only accurately presents actual users, but also their relative importance (i.e. do websites trying to use something fail gracefully, or do I effectively have to use another browser to get achieve what I'm there for).
For example, I use Firefox as my main browser, and the lack of e.g. proper WebAuthN support on macOS has a much, much higher impact to me (and is making me consider switching), whereas I couldn't care less about e.g. WebUSB (which I do occasionally use, but I'm fine switching to Chrome for that use case).
To get an accurate picture, you'd have to somehow weigh all of these various standards and features in a way that not only accurately presents actual users, but also their relative importance (i.e. do websites trying to use something fail gracefully, or do I effectively have to use another browser to get achieve what I'm there for).