I agree with your overall point, but there is a reason sites like MDN exist. It's because some of the HTML/CSS/JS specs are crazy complicated, contain years and years of edge cases and bugs that have become standard, and are written in standard-eze (which is easy enough to read once you know it, but it can be a bit of a learning curve for someone who just wants to know the order of function parameters).
For a more nuanced understanding of browser behavior, you have to read the spec. Worked on a high performance network app, and I think I know the XHR spec by heart now.
I am curious, can you elaborate on this? :) What part of the performance related work involved peeking at the XHR spec so much?