The HTML section of this page provides information for beginners as well as experts on HTML. There are links on the homepage for HTML that take readers to specific pages for each of the most common HTML elements. For beginners, they provide an HTML Basics article to teach about what HTML is and how to use it. They also offer tutorials for how to solve different problems using HTML. Finally, they have an "extensive" HTML reference section that provides information about each element and attribute in HTML. I would use this resource if I am trying to create something with HTML and struggling or if I want to expand my knowledge and use more of the elements in HTML.
Similar to the HTML page, the MDN reference page for CSS provides information for beginners and seasoned developers about CSS. New developers can use the CSS basics article to learn all about CSS and how to get started using it. They also have tutorials on their page to teach how to do new things with CSS. Finally, they have an exhaustive list of CSS properties and concepts in their CSS reference guide. I would use this (and have in the past) to learn how to acheive a specific style with CSS and to learn more about different things I can do with it.
This page contains basic information about as well as links to go more in depth on the Document Object Model. The DOM represents a document on the web as nodes and objects so that programs can interact with the page. This allows scripting languages, like JavaScript, to modify the page using specific methods. For example, developers can use the "querySelectorAll" method to get a list of all elements on the page that meet a given criteria. I would refer to this page to figure out what I can access using the DOM and learn more about how to access different parts of the document using JavaScript.
This page provides an introduction to JavaScript. There is information and links for beginner, intermediate, and advanced developers. In the beginner section, new coders can find answers to basic questions like "what is JavaScript?" and "what can it do?" There is also information about the building blocks of JavaScript as well as JavaScript objects. Furthermore, there is an in depth JavaScript guide with information about the main features of JavaScript. I would use this page to refer to when I want to clarify how something works or learn a new feature/skill in JavaScript.
It is important to have a trusted resource to visit when looking for programming help for several reasons. First, MDN's comprehensive guides provide information and help on just about every thing you could create with HTML, CSS, and more. It is helpful to have one place to go to that will provide answers for all of your questions. Additionally, using a reputable source will ensure that you will be creating valid and safe code. If you use a source with a weaker reputation, you risk creating code that could add malware to help hackers access and control your site, or at least you may end up writing invalid code.