It's like the difference between textbooks and primary literature when it comes to science. When you're researching something, you consult primary literature, b

It's like the difference between textbooks and primary literature when it comes to science. When you're researching something, you consult primary literature, but when you merely want to learn or refresh a subject, a good textbook is much more practical. The same relationship holds for MDN versus WHATWG specs. The latter are more useful when you're implementing parts of the web, or at least dig below the layer that one needs to know to just code websites.