What is missing from this is the ability to toggle a "strict mode" with a browser, or even a "reference implementation browser". Right now developers have to rely on their knowledge alone combined with the simple fact of whether or not their work seems to render correctly on the major browsers. That is where you run afoul of Postel's Maxim, because it should be possible to use these tools (like browsers) in a way that provides the sort of "hey, don't do that" feedback during development even though during normal operation the browser should do its best to make due with whatever it's given.

That same pattern exists elsewhere too, so often people need to do "API science" to figure out how to use various tools. With the common result being the discovery of how to use those tools seemingly effectively, but incorrectly.