The MDN resource for HTML gives a brief overview of the topic. The article lists links for key resources, beginner's tutorials, advanced topics, references and related topics. The side bar on the page also has resources that go more in depth into specific areas of HTML.
The MDN resource for CSS was structured in a similar layout to the HTML version of their resource. It gives a brief overview and introduction to the topic, and provides links for more in-depth and specific aspects of CSS. Like the HTML reference, the side bar on the page has links to guides that cover various areas of CSS.
Overall, you would use these resources if you're learning, or even if you need to brush up on skills you already know. A trusted resource is important when you're looking for programming help because the information you receive has to meet W3C standards. Otherwise you run the risk of writing code that doesn't validate or meet specifications.