The HTML page from MDN explains the introduction to HTML and as well as having references and tutorials on how to use and understand the language. You would use this page if you didn't understand the basics to give yourself a refresher and if you didn't know how something worked, you could read the tutorials on how to write the code.
The CSS page from MDN talks about a language used to describe the presentation of a document such as HTML. The information on this page will teach you the steps of designing your web pages. You would use this page if you wanted to learn how to set different background colors, styles, and more.
In my opinion, a trusted resource can be websites that provide useful tips when it comes to programming help. Using Google to help with your coding can help you learn as long as you take the time to understand what is going on in the code and why it's working the way it is. Learning from other students' codes and modifying it to see how it works has also helped my coding experience in the past.
In conclusion, the MDN has a ton of helpful resources to help guide you about the web.