HTML5 is a living standard, so naturally different browser versions will have different features implemented. Also, there's the thing where you can conform to

HTML5 is a living standard, so naturally different browser versions will have different features implemented. Also, there's the thing where you can conform to a spec but it's still possible to tell a difference in implementation by observing certain things. For example the non cryptographically secure random number generators in chrome and firefox used to be different algorithms. So if you seed the RNG with a number and call it, you used to get different results for firefox and chrome and you could tell if their user agent is lieing. (actually it was a bit more complicated than that, you can't seed Math.random in JS, but you get the point) This isn't about sites needing that info to work correctly. This is what sites do for fingerprinting.