Usually you can find the signal in the first 2 or 3 sentences of in any post.
Medium.com has better SEO than SO and small personal blogs, so it's winning when searching for a solution.
Given the nature of the medium, books usually have the highest guarantee of quality due to the resources used to publish a book.
I won't try to guess why, but the few pages I can access are always high quality, while poor quality pages (as discovered when opening the page anyway in a full browser) always try to block that setup.
I suspect quality means different things to different people and even changes based on context for the same person.
For instance, a blog describing some arcane linux command to run to fix the issue you're having does not need to be the same level of quality as something you're reading to learn + get in depth info about a topic you know at a very shallow level.