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.
The DOM page from MDN explains an introduction to DOM.
You would use this page to get a better understanding of it and how it allows programming languages to interact with web pages.
The Javascript page from MDN explains what javascript is and provides tutorials and references on how to read and write the language.
You would use these resources when you need help understanding the concepts in the language. When looking for programming help, I believe Google is the most reliable and trustworthy resource. Googling how to do something in any programming language will help you learn.
Reading other codes from students seeing why they wrote the code the way they did also helps, but I do believe that you shouldn't just copy code from the internet because you don't
know if it's trustworthy.