Official documentation > StackOverflow.

StackOverflow sometimes has well-grounded and elaborate explanations on a question by well-meaning and knowledgeable developers, but oftentimes the answers also contain outdated or 'dirty' solutions and so it's always to be taken with a grain of salt.

However, in cases like the official Spring documentation, the IETF RFC's and the MDN, those provide a correct explanation to the problem, even if it requires some reading around. When it comes to serious and correct work, I favor these over the bro-zone that is StackOverflow.